Update README and stop posting full changelogs

Git already does quite a good job of keeping a changelog, only
create high-level release notes from now on.

(cherry picked from commit 7a7c62511cc33bb1af87cb10aad73a08910d071b)
Also created release notes for 1.0.

Change-Id: I2ef30a4786d83d5a35788e6f2ad3532ef1aa3da0