Documentation about view in named maps' templates

Closes #287

docs/Map-API.md

@@ -398,6 +398,19 @@ POST /api/v1/map/named
         }
       }
     ]
+  },
+  "view": {
+    "zoom": 4,
+    "center": {
+      "lng": 0,
+      "lat": 0
+    },
+    "bounds": {
+      "west": -45,
+      "south": -45,
+      "east": 45,
+      "north": 45
+    }
   }
 }
 ```
@@ -410,6 +423,16 @@ POST /api/v1/map/named
   - **valid_tokens** when `"method"` is set to `"token"`, the values listed here allow you to instantiate the named map.
 - **placeholders**: Variables not listed here are not substituted. Variables not provided at instantiation time trigger an error. A default is required for optional variables. Type specification is used for quoting, to avoid injections see template format section below.
 - **layergroup**: the layer list definition. This is the MapConfig explained in anonymous maps. See [MapConfig documentation](https://github.com/CartoDB/Windshaft/blob/master/doc/MapConfig-1.1.0.md) for more info.
+- **view** (optional): extra keys to specify the compelling area for the map. It can be used to have a static preview of a named map without having to instantiate it. It is possible to specify it with `center` + `zoom` or with a bounding box `bbox`. Center+zoom takes precedence over bounding box.
+  - **zoom** The zoom level to use
+  - **center**
+    - **lng** The longitude to use for the center
+    - **lat** The latitude to use for the center
+  - **bounds**
+    - **west**: The western point for the bounding box
+    - **south**: The southern point for the bounding box
+    - **east**: The eastern point for the bounding box
+    - **north**: The northern point for the bounding box
 
 #### Template Format