Update advisory format and introduce some automation

Simplify the advisory format by dropping the -Backport tags and instead
stick to using just the -Commit tags.  To identify backports, put a
substring of git-describe into the release version in the brackets next
to the commit ref.  This way, it not only identifies that the fix (or
regression) is on the release/2.YY/master branch, it also disambiguates
regressions/fixes in the branch from those in the tarball.

Add a README to make it easier for consumers to understand the format.
Additionally, the Release wiki needs to be updated to inform the release
manager to:

1. Generate a NEWS snipped from the advisories directory

AND

2. on release/2.YY/master, replace the advisories directory with a text
   file pointing to the advisories directory in master so that we don't
   have to update multiple locations.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Andreas K. Hüttel <dilfridge@gentoo.org>