Update README with official Git repository.

We are now about to publish this module on the Linköping University
GitLab instance, https://gitlab.liu.se/NSC/puppetmodules/cfgfile.git

README

 This is a Puppet module for editing and managing generic config files.
-Copyright © 2008-2019 its various authors.
+Copyright © 2008-2020 its various authors.
 
 
 ===== LICENSING =====
@@ -23,3 +23,7 @@ If not, see <http://www.gnu.org/licenses/>.
 ===== DEPENDENCIES =====
 
 
+===== OFFICIAL SOURCE =====
+
+The official Git repository for this module is located at
+https://gitlab.liu.se/NSC/puppetmodules/cfgfile.git