[rygel] docs: RygelMediaEngine: Suggest transcoder implementation.

[Murray Cumming <murrayc src gnome org>]

commit 38d7887d40f388818dc47e311311729d17c29704
Author: Murray Cumming <murrayc openismus com>
Date:   Mon Feb 11 11:33:14 2013 +0100

    docs: RygelMediaEngine: Suggest transcoder implementation.
    
    Added some text from Mark Ryan.

 src/librygel-server/rygel-media-engine.vala |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletions(-)
---
diff --git a/src/librygel-server/rygel-media-engine.vala b/src/librygel-server/rygel-media-engine.vala
index bbbd268..3c55cd6 100644
--- a/src/librygel-server/rygel-media-engine.vala
+++ b/src/librygel-server/rygel-media-engine.vala
@@ -37,9 +37,21 @@ public errordomain Rygel.MediaEngineError {
  * For instance, in rygel.conf:
  * media-engine=librygel-media-engine-gst.so
  *
- * Media engines should also derive their own RygelDataSource,
+ * Media engines should also derive their own #RygelDataSource,
  * returning an instance of it from create_data_source().
  *
+ * If this media engine supports transcoding then it will typically
+ * implement a set of transcoding classes, typically with one 
+ * base class and a number of sub-classes - one for each transcoding
+ * format you want to support. These should be returned by the
+ * get_transcoders() virtual function. The base transcoder class could
+ * provide a generic way to create a #RygelDataSource capable of
+ * providing Rygel with a transcoded version of a file using the
+ * underlying media framework. The sub-classes could contain the
+ * various media-framework-specific parameters required to 
+ * transcode to a given format and implement a heuristic that
+ * can be used to order an item's transcoded resources.
+ *
  * See the
  * <link linkend="implementing-media-engines">Implementing Media Engines</link> section.
  */