doc: vapoursynth

Add entry in external library list and demuxers.

doc/demuxers.texi

@@ -732,4 +732,20 @@ Example: convert the captions to a format most players understand:
 ffmpeg -i http://www.ted.com/talks/subtitles/id/1/lang/en talk1-en.srt
 @end example
 
+@section vapoursynth
+
+Vapoursynth wrapper.
+
+Due to security concerns, Vapoursynth scripts will not
+be autodetected so the input format has to be forced. For ff* CLI tools,
+add @code{-f vapoursynth} before the input @code{-i yourscript.vpy}.
+
+This demuxer accepts the following option:
+@table @option
+@item max_script_size
+The demuxer buffers the entire script into memory. Adjust this value to set the maximum buffer size,
+which in turn, acts as a ceiling for the size of scripts that can be read.
+Default is 1 MiB.
+@end table
+
 @c man end DEMUXERS

doc/general.texi

@@ -242,6 +242,17 @@ Go to @url{http://www.twolame.org/} and follow the
 instructions for installing the library.
 Then pass @code{--enable-libtwolame} to configure to enable it.
 
+@section VapourSynth
+
+FFmpeg can read VapourSynth scripts as input. To enable support, pass
+@code{--enable-vapoursynth} to configure. Vapoursynth is detected via
+@code{pkg-config}. Versions 42 or greater supported.
+See @url{http://www.vapoursynth.com/}.
+
+Due to security concerns, Vapoursynth scripts will not
+be autodetected so the input format has to be forced. For ff* CLI tools,
+add @code{-f vapoursynth} before the input @code{-i yourscript.vpy}.
+
 @section WavPack
 
 FFmpeg can make use of the libwavpack library for WavPack encoding.