doc: add documentation regarding MIPS optimizations

Add doc/mips.txt document with short explanation regarding targeted
platform for MIPS optimizations and regarding MIPS copyright notice.
Signed-off-by: Nedeljko Babic <nbabic@mips.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/mips.txt

+MIPS optimizations info
+===============================================
+
+MIPS optimizations of codecs are targeting MIPS 74k family of
+CPUs. Some of these optimizations are relying more on properties of
+this architecture and some are relying less (and can be used on most
+MIPS architectures without degradation in performance).
+
+Along with FFMPEG copyright notice, there is MIPS copyright notice in
+all the files that are created by people from MIPS Technologies.
+
+Example of copyright notice:
+===============================================
+/*
+ * Copyright (c) 2012
+ *      MIPS Technologies, Inc., California.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the MIPS Technologies, Inc., nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE MIPS TECHNOLOGIES, INC. ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE MIPS TECHNOLOGIES, INC. BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * Author:  Author Name (author_name@@mips.com)
+ */
+
+Files that have MIPS copyright notice in them:
+===============================================
+* libavutil/mips/
+      libm_mips.h
+* libavcodec/mips/
+      acelp_filters_mips.c
+      acelp_vectors_mips.c
+      amrwbdec_mips.c
+      amrwbdec_mips.h
+      celp_filters_mips.c
+      celp_math_mips.c
+      lsp_mips.h
+      dsputil_mips.c
+      fft_mips.c
+      fft_table.h
+      fmtconvert_mips.c