Add documentation for option

README.md

@@ -137,6 +137,14 @@ Value: a string (which can be resolved to an object type on the global scope), o
 
 Determine the type of collections used for a `HasMany` relation. Defining a `url(models<Backbone.Model[]>)` function on this Collection that's able to build a url for either the whole collection, or a set of models enables `fetchRelated` to fetch all missing models in one request, instead of firing a separate request for each. See [Backbone-tastypie](https://github.com/PaulUithol/backbone-tastypie/blob/master/backbone-tastypie.js#L74) for an example.
 
+### collectionKey
+
+Value: a string or a boolean
+
+By default, the relation's `key` attribute will be used to create a reference to the RelationalModel instance from the generated collection. If you set `collectionKey` to a string, it will use that string as the reference to the RelationalModel, rather than the relation's `key` attribute. Be careful: if you use a value for `key` or `collectionKey` that is the name of an already-existing property on the RelationalModel, that property will be overwritten by the reference to the RelationalModel instance.
+
+If you don't want this behavior at all, just set `collectionKey` to false (or any falsy value) and this reference will not be created.
+
 ### includeInJSON
 
 Value: a boolean, or a string referencing one of the model's attributes. Default: `true`.