documentation: add encoders chapter

Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>
(cherry picked from commit 0fa904c9)

doc/encoders.texi

+@chapter Encoders
+@c man begin ENCODERS
+
+Encoders are configured elements in FFmpeg which allow the encoding of
+multimedia streams.
+
+When you configure your FFmpeg build, all the supported native encoders
+are enabled by default. Encoders requiring an external library must be enabled
+manually via the corresponding @code{--enable-lib} option. You can list all
+available encoders using the configure option @code{--list-encoders}.
+
+You can disable all the encoders with the configure option
+@code{--disable-encoders} and selectively enable / disable single encoders
+with the options @code{--enable-encoder=@var{ENCODER}} /
+@code{--disable-encoder=@var{ENCODER}}.
+
+The option @code{-codecs} of the ff* tools will display the list of
+enabled encoders.
+
+@c man end ENCODERS

doc/ffmpeg.texi

@@ -981,6 +981,7 @@ file to which you want to add them.
 @c man end EXAMPLES
 
 @include eval.texi
+@include encoders.texi
 @include demuxers.texi
 @include muxers.texi
 @include indevs.texi