DEVELOPING.md, CONTRIBUTING.md: Include them in tarball and installation

Added the new files to Makefile.am so that "make dist" amd "make
install" include them.

Author: tillkamppeter

Makefile.am

@@ -12,7 +12,9 @@ doc_DATA = \
 	CHANGES.md \
 	CHANGES-1.x.md \
 	INSTALL \
-	README.md
+	README.md \
+	DEVELOPING.md \
+	CONTRIBUTING.md
 
 EXTRA_DIST = \
 	$(doc_DATA) \