README: Major update

* convert to markdown syntax
* bring in-line with other osmo-* README.md files, in terms of
  links to git, mailing list, manuals, etc.

Change-Id: Ia4a4329c6ef6b8c833aa26832776dad662cdc7e9
diff --git a/contrib/osmo-sgsn.spec.in b/contrib/osmo-sgsn.spec.in
index 9cf18f6..daedf06 100644
--- a/contrib/osmo-sgsn.spec.in
+++ b/contrib/osmo-sgsn.spec.in
@@ -93,7 +93,7 @@
 make %{?_smp_mflags} check || (find . -name testsuite.log -exec cat {} +)
 
 %files
-%doc AUTHORS README
+%doc AUTHORS README.md
 %dir %{_docdir}/%{name}/examples
 %dir %{_docdir}/%{name}/examples/osmo-sgsn
 %exclude %{_docdir}/%{name}/examples/osmo-gtphub