Move RELEASE_NOTES into the docs subdirectory

Move RELEASE_NOTES into the docs subdirectory

We will begin maintaining release notes inside the source directory,
with one RELEASE_NOTES.txt file per major release series (2.2, 2.3,
etc). The release notes will primarily consist of an overview of new
features and important behaviour changes in that particular release
series, as well as a cumulative list of bugs that have been fixed in the
point releases for that series.

Also, mention the existence of the release notes in the top-level
README.

Signed-off-by: Dan Scott <dscott@laurentian.ca>