Rename RELEASE_NOTES.txt to RELEASE_NOTES.md

Renaming RELEASE_NOTES.txt to RELEASE_NOTES.md will allow
source code browsers (like googlesource.com) to render the
Markdown as formatted text instead of the raw Markdown text.

Bug: skia:14181
Change-Id: Ic08efdec258d347f2b61827793cdaa63f71beb9a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/663376
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Chris Mumford <cmumford@google.com>
4 files changed