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.

Reported-by: Flavio Leitner <fbl@redhat.com>
Reported-at: http://openvswitch.org/pipermail/dev/2014-June/040952.html
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Flavio Leitner <fbl@redhat.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>