Introduce a description of the Backport and Fixes tags

Introduce a description of the Backport and Fixes tags

Create a new document under docs/process to describe our special tags.
Add a description of the Fixes tag and the new Backport tag. Also
clarify that lines with tags should not be split.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Wei Liu <wl@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
CC: jbeulich@suse.com
CC: george.dunlap@citrix.com
CC: julien@xen.org
CC: lars.kurth@citrix.com
CC: andrew.cooper3@citrix.com
CC: konrad.wilk@oracle.com