add an API changelog with a first entry for the new metadata API

Originally committed as revision 17697 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

+20090301 - r17682 - lavf 52.31.0 - Generic metadata API
+  This version introduce a new metadata API (see av_metadata_get() and friends).
+  The old API is now deprecated and shouldn't be used anymore. This especially
+  include the following structure fields:
+    - AVFormatContext.title
+    - AVFormatContext.author
+    - AVFormatContext.copyright
+    - AVFormatContext.comment
+    - AVFormatContext.album
+    - AVFormatContext.year
+    - AVFormatContext.track
+    - AVFormatContext.genre
+    - AVStream.language
+    - AVStream.filename
+    - AVProgram.provider_name
+    - AVProgram.name
+    - AVChapter.title