Add a part explaining that API should be considered unstable after major version changes and list dates of last major version increase.
Add a part explaining that API should be considered unstable after major
version changes and list dates of last major version increase.

git-svn-id: a4d7c1866f8397a4106e0b57fc4fbf792bbdaaaf@17878 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg