Add dependency of contao-leaflet-libraries

assets/leaflet-providers/CONTRIBUTING.md

@@ -0,0 +1,10 @@
+So you want to add a layer?
+=======
+
+Yay! go add it to the leaflet-providers.js as long as it follows the following 
+rules:
+
+- Don't violate a providers TOS (if it exists, include a link to it)
+- Don't pre-populate api keys with working keys.
+- It should be a basic tile source, no exteral libraries etc.
+- The owner hasn't asked us to remove it (hasn't happened yet)