Switch to putting new release notes in //relnotes/

To avoid common merge conflicts new release notes will now
be added, as new files, in the //relnotes directory. As part
of the release process a utility will be run that aggregates
these notes and modifies RELEASE_NOTES.md.

## Background
Previously when developers make a change worth note in
the release notes, they usually add a new list item in the
unordered list in the next milestone section in //RELEASE_NOTES.md.
This produces merge conflicts in local branches (with a modified
RELEASE_NOTES.md) when syncing with the origin repo where
RELEASE_NOTES.md was modified in another change. This conflict
arises because both changes have modified the same line of that
file.

Design: https://docs.google.com/document/d/1JEDWt4MgtbTt8IdMvOkqMEA5lVakXGtxGeoyHaXVeoA

Bug: skia:10798
Change-Id: I97e954e0c7bfb4d70ec11d27cde3ec0a6b9172bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/668416
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed