doc/encoders: add basic libx265 documentation

Fixes ticket #3944.
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/encoders.texi

@@ -1886,6 +1886,34 @@ no-fast-pskip=1:subq=6:8x8dct=0:trellis=0 OUTPUT
 Encoding ffpresets for common usages are provided so they can be used with the
 general presets system (e.g. passing the @option{pre} option).
 
+@section libx265
+
+x265 H.265/HEVC encoder wrapper.
+
+This encoder requires the presence of the libx265 headers and library
+during configuration. You need to explicitly configure the build with
+@option{--enable-libx265}.
+
+@subsection Options
+
+@table @option
+@item preset
+Set the x265 preset.
+
+@item tune
+Set the x265 tune parameter.
+
+@item x265-params
+Set x265 options using a list of @var{key}=@var{value} couples separated
+by ":". See @command{x265 --help} for a list of options.
+
+For example to specify libx265 encoding options with @option{-x265-params}:
+
+@example
+ffmpeg -i input -c:v libx265 -x265-params crf=26:psy-rd=1 output.mp4
+@end example
+@end table
+
 @section libxvid
 
 Xvid MPEG-4 Part 2 encoder wrapper.