Adds support for top-level links to JsonApi adapter

http://jsonapi.org/format/#document-top-level

fix failing tests

support for top-level links limited to jsonapi adapter

Move docs from README to docs/ dir

move links to json-api adapter & create Links class to hold links data

docs/README.md

@@ -23,6 +23,7 @@ This is the documentation of ActiveModelSerializers, it's focused on the **0.10.
 - [How to add pagination links](howto/add_pagination_links.md)
 - [Using ActiveModelSerializers Outside Of Controllers](howto/outside_controller_use.md)
 - [Testing ActiveModelSerializers](howto/test.md)
+- [How to add top-level links](howto/add_top_level_links.md) (```JSON-API``` only)
 
 ## Integrations
 

docs/howto/add_top_level_links.md

@@ -0,0 +1,31 @@
+# How to add top-level links
+
+JsonApi supports a [links object](http://jsonapi.org/format/#document-links) to be specified at top-level, that you can specify in the `render`:
+
+```ruby
+  render json: @posts, links: { "self": "http://example.com/api/posts" }
+```
+
+That's the result:
+
+```json
+{
+  "data": [
+    {
+      "type": "posts",
+      "id": "1",
+      "attributes": {
+        "title": "JSON API is awesome!",
+        "body": "You should be using JSON API",
+        "created": "2015-05-22T14:56:29.000Z",
+        "updated": "2015-05-22T14:56:28.000Z"
+      }
+    }
+  ],
+  "links": {
+    "self": "http://example.com/api/posts"
+  }
+}
+```
+
+This feature is specific to JsonApi, so you have to use the use the [JsonApi Adapter](https://github.com/rails-api/active_model_serializers/blob/master/docs/general/adapters.md#jsonapi)