add to dist: doc subdir with all current doc files

Change-Id: Idfef4f050e21a5d0eae3a530fe319a5adf81534d
diff --git a/Makefile.am b/Makefile.am
index c2167d4..ab8191e 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,6 +1,6 @@
 AUTOMAKE_OPTIONS = foreign dist-bzip2
 
-SUBDIRS = src include
+SUBDIRS = src include doc
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libosmo-ranap.pc