Add a release note for 2.3

Based on a patch by Clément Bœsch <u@pkh.me>.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

RELEASE_NOTES

+ ┌─────────────────────────────────┐
+ │ RELEASE NOTES for FFmpeg 2.3    │
+ └─────────────────────────────────┘
+
+   The FFmpeg Project proudly presents FFmpeg 2.3, a major release with all
+   the great features committed during the three-month period since the
+   release of FFmpeg 2.2.
+
+   In this release, there are lots of internal overhauls that make FFmpeg a
+   more accessible project for new developers. Many important new features
+   like native Opus decoder and QTKit and AVFoundation input devices are also
+   committed.
+
+   Because of the increasing difficulty to maintain and lack of maintainers,
+   we are very sorry to say that we have removed all Blackfin and SPARC
+   architecture assembly optimizations with the cleanups done. If you are
+   interested in maintaining optimization for these two architecture, feel
+   free to contact us and we will restore the code!
+
+   Since this release, the traditional Changelog file is upgraded to this
+   modern-looking release note. Old changelogs are moved to doc/Changelog.old.
+
+   Enjoy!
+
+   ┌────────────────────────────┐
+   │ *  API Information         │
+   └────────────────────────────┘
+
+     FFmpeg 2.3 is completely source-compatible to the FFmpeg 2.2 series. There
+     are however some API deprecations that you need to take care of. Use `git
+     diff n2.2 n2.3 doc/APIchanges` to show the list of added and deprecated
+     APIs. FFmpeg 2.3 includes the following library versions:
+
+       • libavutil      xx.yy.1zz
+       • libavcodec     xx.yy.1zz
+       • libavformat    xx.yy.1zz
+       • libavdevice    xx.yy.1zz
+       • libavfilter    xx.yy.1zz
+       • libswscale     xx.yy.1zz
+       • libswresample  xx.yy.1zz
+       • libpostproc    xx.yy.1zz
+
+     Please refer to the doc/APIChanges file for more information.
+
+   ┌────────────────────────────┐
+   │ New Optimization           │
+   └────────────────────────────┘
+
+     We are excited to announce that we have committed new x86 assembly
+     optimization for HEVC, and FFmpeg's audio resampler libswresample. ARM
+     users will get a boost in MLP/TrueHD decoding thanks to new optimization.
+     Decoding Huffyuv also got a major boost from optimization on the C code.
+
+     Of special interest for Microsoft Visual Studio users, we have also
+     converted some preexisting x86 assembly to NASM/Yasm format compatible
+     with MSVC setup, especially in the area of audio resampling.
+
+     Another major feature in this release is the introduction of AArch64
+     (ARMv8) assembly optimization. AArch64 is another name for the first
+     64-bit ARM architecture, used by Apple A7 SoC inside iPhone 5S. We have
+     already ported some 32-bit ARM to AArch64, but more work is underway.
+
+   ┌────────────────────────────┐
+   │ Native Opus decoder        │
+   └────────────────────────────┘
+
+     Opus is an open audio format jointly developed by Xiph.Org, Mozilla,
+     Skype/Microsoft, and Broadcom. It combines the features of the Skype Cilk
+     speech codec and the Xiph.Org CELT music codec into one low-latency
+     codec. Decoding Opus is already possible since FFmpeg 1.0 using the
+     libopus library, but the new Opus native decoder brings a higher level of
+     stability and speed.
+
+   ┌────────────────────────────┐
+   │ QTKit and AVFoundation     │
+   └────────────────────────────┘
+
+     For OS X users, the new QTKit and AVFoundation devices allow you to use
+     the integrated camera on Macs. AVFoundation is a newer API only available
+     on OS X 10.7 "Lion" or newer. For users with older OS X systems, the
+     QTKit device using the older OS X API is for you.
+
+   ┌────────────────────────────┐
+   │ API Additions              │
+   └────────────────────────────┘
+
+     In this release, stream side data are introduced as AVStream.side_data as
+     a way to store miscellaneous stream-wide information. The format is
+     similar to the previously anonymous structure AVPacket.side_data (now
+     named as AVPacketSideData). With this change, audio ReplayGain
+     information and video rotation matrix are now exported through this API,
+     if available in the demuxer.
+
+     We also have improved libswresample's Doxygen API documentation, so new
+     developers wishing to use FFmpeg's excellent libraries can get started
+     more easily and faster.
+
+   ┌────────────────────────────┐
+   │ Last But Not Least         │
+   └────────────────────────────┘
+
+     Other interesting new features including hqx video filter, a pixel art
+     scaling filter; a fixed-point AC-3 decoder contributed by Imagination
+     Technologies; an On2 TrueMotion VP7 video decoder; an HTML5 WebVTT
+     subtitle decoder that allows creation of WebVTT from any text-based
+     subtitles; and an 1-bit Direct Stream Digital audio decoder.
+
+ ┌────────────────────────────┐
+ │ ★  List of New Features    │
+ └────────────────────────────┘
+
+   ┌────────────────────────────┐
+   │ Command line tools         │
+   └────────────────────────────┘
+
+    • Support for decoding through DXVA2 in ffmpeg
+
+   ┌────────────────────────────┐
+   │ libavcodec                 │
+   └────────────────────────────┘
+
+    • AC3 fixed-point decoding
+    • VP7 video decoder
+    • Alias PIX image encoder and decoder
+    • Improvements to the BRender PIX image decoder
+    • Improvements to the XBM decoder
+    • Improvements to OpenEXR image decoder
+    • Support decoding 16-bit RLE SGI images
+    • Direct Stream Digital (DSD) decoder
+    • On2 AVC (Audio for Video) decoder
+    • Native Opus decoder
+    • WebVTT encoder
+
+   ┌────────────────────────────┐
+   │ libavdevice                │
+   └────────────────────────────┘
+
+    • QTKit input device
+    • GDI screen grabbing for Windows
+    • AVFoundation input device
+
+   ┌────────────────────────────┐
+   │ libavformat                │
+   └────────────────────────────┘
+
+    • subfile protocol
+    • Phantom Cine demuxer
+    • Alternative rendition support for HTTP Live Streaming
+    • Magic Lantern Video (MLV) demuxer
+    • Image format auto-detection
+    • LRC lyric file demuxer and muxer
+    • Samba protocol (via libsmbclient)
+
+   ┌────────────────────────────┐
+   │ libavfilter                │
+   └────────────────────────────┘
+
+    • shuffleplanes filter
+    • libbs2b-based stereo-to-binaural audio filter
+    • showcqt multimedia filter
+    • zoompan filter
+    • signalstats filter
+    • hqx filter (hq2x, hq3x, hq4x)
+    • flanger filter
+
+ ┌────────────────────────────┐
+ │ ⚠  Behaviour changes       │
+ └────────────────────────────┘
+
+  • libx264 reference frames count is now limited depending on level chosen
+  • Because of the new image format auto-detection feature, you don't need to
+    specify image format when decoding an image with no extension.