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/Makefile.am b/Makefile.am
index e524271..045e74d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -24,7 +24,7 @@
 BUILT_SOURCES = $(top_srcdir)/.version
 EXTRA_DIST = \
 	     .version \
-	     README \
+	     README.md \
 	     contrib/osmo-mgw.spec.in \
 	     debian \
 	     git-version-gen \