Release notes: Maintain 1.20 notes in master, revert history.

Release notes: Maintain 1.20 notes in master, revert history.

* The one in history isn't final yet, which makes maintenance
  harder as it would have to be synced between 2 different file
  names across 2 different branches.

* For backport commits its easier if the 1.20 file is still in
  master so that they can be written together with the commit
  (and so that the history is in master as well), as opposed to
  merging it in master without notes and then adding them later
  on in the (amended) merge version.

Change-Id: I3fa4b88a5845a51e83221d78facaba1ddb3b1acb