README.md: link to RFE about systemd-homed fscrypt version support

Including a link to the specific report can help a reader recognize
when the warning about systemd-homed is no longer important.

It might also help to drive attention to the right place to improve
systemd-homed.

Co-authored-by: Joe Richey <joerichey@google.com>
Co-authored-by: <dkg@fifthhorseman.net>

This warning is only applicable for systems with kernel versions older
than v4.1, which is now too old to be worth the mention here.

With the latest version of go, the recommended 'go get' commands fail:

    go: go.mod file not found in current directory or any parent directory.
            'go get' is no longer supported outside a module.
            To build and install a command, use 'go install' with a version,
            like 'go install example.com/cmd@latest'
            For more information, see https://golang.org/doc/go-get-install-deprecation
            or run 'go help get' or 'go help install'.

Replace them with commands that work.

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.

Now that it's been requested by users, bring back the "unlock_only"
option, which was originally proposed as part of
https://github.com/google/fscrypt/pull/281 but was dropped in the final
version of that pull request.

Resolves https://github.com/google/fscrypt/issues/357