Copy the GitHub release notes into a NEWS.md file so that the release
notes are included in the actual git repo.  This way, they aren't hidden
away in GitHub, where they require an internet connection to access and
will be lost if GitHub ever goes away.  This also makes the release
notes be properly versioned; GitHub allows past release notes to be
edited, and there doesn't seem to be any record of what changed.

Finally, this allows packages to install the release notes into
/usr/share/doc/$pkgname/, as is the usual convention.