doc/encoders: add libopenh264 entry

doc/encoders.texi

@@ -1342,6 +1342,64 @@ disabled
 A description of some of the currently available video encoders
 follows.
 
+@section libopenh264
+
+Cisco libopenh264 H.264/MPEG-4 AVC encoder wrapper.
+
+This encoder requires the presence of the libopenh264 headers and
+library during configuration. You need to explicitly configure the
+build with @code{--enable-libopenh264}. The library is detected using
+@command{pkg-config}.
+
+For more information about the library see
+@url{http://www.openh264.org}.
+
+@subsection Options
+
+The following FFmpeg global options affect the configurations of the
+libopenh264 encoder.
+
+@table @option
+@item b
+Set the bitrate (as a number of bits per second).
+
+@item g
+Set the GOP size.
+
+@item maxrate
+Set the max bitrate (as a number of bits per second).
+
+@item flags +global_header
+Set global header in the bitstream.
+
+@item slices
+Set the number of slices, used in parallelized encoding. Default value
+is 0. This is only used when @option{slice_mode} is set to
+@samp{fixed}.
+
+@item slice_mode
+Set slice mode. Can assume one of the follwing possible values:
+
+@table @samp
+@item fixed
+a fixed number of slices
+@item rowmb
+one slice per row of macroblocks
+@item auto
+automatic number of slices according to number of thread
+@end table
+
+Default value is @samp{auto}.
+
+@item loopfilter
+Enable loop filter, if set to 1 (automatically enabled). To disable
+set a value of 0.
+
+@item profile
+Set profile restrictions. If set to the value of @samp{main} enable
+CABAC (set the @code{SEncParamExt.iEntropyCodingModeFlag} flag to 1).
+@end table
+
 @section jpeg2000
 
 The native jpeg 2000 encoder is lossy by default, the @code{-q:v}