CONTRIBUTING: Describe commonly used tags; introduce Reported-at.
This is partly documentation of how patches are tagged in practice in Open
vSwitch. The bits at the end about "Reported-at:" and "VMware-BZ:" are
an attempt to influence future practices; I cannot say how successful they
will be.
I am not sure whether these key-value pairs at the end of commit messages
are actually commonly called "tags". I'm happy to use a different term if
that one seems wrong.