Release notes: Get back lost history

Release notes: Get back lost history

REL1_20 was branched from master as it was on 2012-09-15, but
release notes was removed from latest master. So there's stuff
missing.

I copied over parts from this diff:

$ git diff gerrit/REL1_20...e4f62bbb0b393e7f01ee5ee8390e1c5e890f7ead -- RELEASE-NOTES-1.20

Fixed existing entries, and copying new entries to
RELEASE-1.21 as they never made it to 1.20, because we branched
REL1_20 from a past master.

Change-Id: Ic07c57c4b3fb27074ea8a0d85b56837f8db81bd1