Commit 9428c28f7a37038b594c5ff36b1669b0f21afe41 added a note in
`CONTRIBUTING.md` saying that, "For consistency, there should not be a
period at the end of the commit message."
I believe it would be more correct (with respect to consistency) to
say that there should not be a full stop/period at the end of the
commit message's *summary line* (the first line of the commit message)
(indeed, the example commit message just above this note clearly does
use full stops outside of its summary line), and this patch changes
`CONTRIBUTING.md` accordingly.
Suggest prefixing commit messages that touches NixOS code with
"nixos/<module>" instead of the current "<name> service" prefix.
"<name> service" is limiting in that NixOS code is more than the
services. It is also easier to spot NixOS changes with explicit
"nixos/..." prefix.
