Convert README to README.md and expand like in other projects

Let's align with structure and content of README.md in other osmo-*
projects.

Change-Id: Id9a78ccc0d10f0e3a9d832ac4c4988cfbb8ddb3c
diff --git a/contrib/osmo-mgw.spec.in b/contrib/osmo-mgw.spec.in
index fa08acd..a7963a3 100644
--- a/contrib/osmo-mgw.spec.in
+++ b/contrib/osmo-mgw.spec.in
@@ -99,7 +99,7 @@
 
 %files
 %license COPYING
-%doc AUTHORS README
+%doc AUTHORS README.md
 %dir %{_docdir}/%{name}/examples
 %dir %{_docdir}/%{name}/examples/osmo-mgw
 %{_docdir}/%{name}/examples/osmo-mgw/osmo-mgw.cfg