build: include README.md into the release tarball

Change-Id: Icef2122de065683ee6a18f507c1834066c950349
diff --git a/Makefile.am b/Makefile.am
index 3861d29..192ceeb 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,6 +19,7 @@
 BUILT_SOURCES = $(top_srcdir)/.version
 EXTRA_DIST = \
 	     .version \
+	     README.md \
 	     contrib/osmo-smlc.spec.in \
 	     debian \
 	     git-version-gen \