feat(documentation): Added the layers attribute documentation

doc/layers-attribute.md

@@ -0,0 +1,35 @@
+'layers' Attribute Documentation
+==================================
+
+This sub-directive needs the **leaflet** main directive, so it is normaly used as an attribute of the *leaflet* tag, like this:
+
+```
+<leaflet layers="layers"></leaflet>
+```
+
+It will map an object _layers_ of our controller scope with the corresponding object on our directive isolated scope. It's not a bidirectional relationship, so only the changes made in this object on the controller scope will affect the map center position. Let's define the layers model with an example:
+
+```
+$scope.layers = {
+    layers: {
+        baselayers: {
+            osm: {
+                name: 'OpenStreetMap',
+                url: 'http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png',
+                type: 'xyz'
+            },
+            cloudmade: {
+                name: 'Cloudmade Tourist',
+                type: 'xyz',
+                url: 'http://{s}.tile.cloudmade.com/{key}/{styleId}/256/{z}/{x}/{y}.png',
+                layerParams: {
+                    key: '007b9471b4c74da4a6ec7ff43552b16f',
+                    styleId: 7
+                }
+            }
+        }
+    }
+}
+```
+
+We can see that the _layers_ definition is conformed by lot attributes that we are going to describe belo. When we associate that object with our _leaflet-directive_ the unidirectional relation will start, and the first layer will be shown on the map and a new layer-switcher-control will appear on our map.

doc/leaflet-directive.md

@@ -90,3 +90,5 @@ We have much more possibilities than showing a simple map, but this will need th
 * [_center_ attribute](https://github.com/tombatossals/angular-leaflet-directive/blob/master/doc/center-attribute.md)
 * [_bounds_ attribute](https://github.com/tombatossals/angular-leaflet-directive/blob/master/doc/bounds-attribute.md)
 * [_maxbounds_ attribute](https://github.com/tombatossals/angular-leaflet-directive/blob/master/doc/maxbounds-attribute.md)
+* [_tiles_ attribute](https://github.com/tombatossals/angular-leaflet-directive/blob/master/doc/tiles-attribute.md)
+* [_layers_ attribute](https://github.com/tombatossals/angular-leaflet-directive/blob/master/doc/layers-attribute.md)