Add changelog entry for video thumbnail fix

Document the fix for video thumbnail generation failing on some
MP4/MOV files when ffmpeg reads from a non-seekable stdin pipe.

Author: dahlia

CHANGES.md

@@ -6,6 +6,13 @@ Version 0.7.7
 
 To be released.
 
+ -  Fixed video thumbnail generation failing for some MP4/MOV files by writing
+    the video data to a temporary file instead of piping it via stdin (`pipe:0`),
+    which does not support seeking.  [[#397], [#398] by NTSK]
+
+[#397]: https://github.com/fedify-dev/hollo/issues/397
+[#398]: https://github.com/fedify-dev/hollo/pull/398
+
 
 Version 0.7.6
 -------------