Add developer tools submodule

This is intended for utilities to help with development,
which aren't needed for building.

See https://wiki.blender.org/index.php/Dev:Doc/Tools/Blender_Tools_Repo for details

.gitmodules

@@ -10,3 +10,7 @@
 	path = release/datafiles/locale
 	url = ../blender-translations.git
 	ignore = all
+[submodule "source/tools"]
+	path = source/tools
+	url = ../blender-dev-tools.git
+	ignore = all