install: Run GPM.

Suggested by Adam Pribyl <pribyl@lowlevel.cz>
at <https://lists.gnu.org/archive/html/guix-devel/2015-11/msg00133.html>.

* gnu/system/install.scm (installation-services): Add call to
  'gpm-service'.
* doc/guix.texi (System Installation): Mention GPM.