Add .gitreview similar to the other osmocom's projects

A gitreview file is required to use git review.
More information about git review
https://docs.openstack.org/infra/git-review/
https://www.mediawiki.org/wiki/Gerrit/git-review

Change-Id: Ie07446ba1a13e53c87bcc9b23e3b775803d158f4
diff --git a/.gitreview b/.gitreview
new file mode 100644
index 0000000..85e5df7
--- /dev/null
+++ b/.gitreview
@@ -0,0 +1,3 @@
+[gerrit]
+host=gerrit.osmocom.org
+project=gr-gsm