build: include README.md into the release tarball

Change-Id: I1fcd14371dd2576f45c9c0798fd5c3628b631e13
diff --git a/Makefile.am b/Makefile.am
index 3040616..f793b34 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -5,6 +5,7 @@
 BUILT_SOURCES = $(top_srcdir)/.version
 EXTRA_DIST = \
 	     .version \
+	     README.md \
 	     contrib/osmo-pcap.spec.in \
 	     debian \
 	     git-version-gen \