Add multilayer api file as internal doc

docs/internal/multilayer-API.md

@@ -0,0 +1,28 @@
+The Windshaft-CartoDB MultiLayer API extends the [Windshaft MultiLayer API](https://github.com/CartoDB/Windshaft/blob/master/doc/internal/multilayer-API.md) in a few ways.
+
+## Last modification timestamp embedded in the token
+
+It encodes a timestamp of 'last modification time' into the map token (token:EPOCH) returned to the client.
+It accepts tokens with encoded timestamp from the client considering the token suffix as a cache_buster value.
+
+Clients don't need to be aware of the extension but rather use the API as they would use the base one.
+The only difference will be that the _same_ layergroup configuration may result in different tokens if source data was modified between the mapview requests. 
+
+## Additional attributes in the response object
+
+Windshaft-CartoDB adds the following attributes in the response object
+
+- ``last_update`` field with ISO format (2013-11-30T12:23:10).
+- ``cdn_url`` object containing CDN url client should use (not mandatory) to access the tiles. It's in the form:
+
+   ```json
+   {
+     "http": "http://cdn_url.com/",
+     "https": "https://secure.cdn_url.com/"
+   }
+   ```
+
+
+## Stats tag
+
+Windshaft-CartoDB adds support for a ``stat_tag`` element in the multilayer configuration to help [stats](https://github.com/CartoDB/Windshaft-cartodb/wiki/Redis-stats-format) gathering.