Update Changelog for v10.1

Changelog

 Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
+version 10.1:
+- rtmpproto: Check the buffer sizes when copying app/playpath strings
+- swscale: Fix an undefined behaviour
+- vp9: Read the frame size as unsigned
+- dcadec: Use correct channel count in stereo downmix check
+- dcadec: Do not decode the XCh extension when downmixing to stereo
+- matroska: add the Opus mapping
+- matroskadec: read the CodecDelay element
+- rtmpproto: Make sure to pass on the error code if read_connect failed
+- lavr: allocate the resampling buffer with a positive size
+- mp3enc: Properly write bitrate value in XING header (debian/736088)
+- golomb: Fix the implementation of get_se_golomb_long
+
 version 10:
 - af_channelmap: fix ONE_STR mapping mode
 - matroskaenc: HEVC fixeswrite private data in hvcC format for HEVC.