Adds support for meta attribute

Currently, 0.10.0.pre doesn't support `meta` option in `render`. This
way, there's no way to support features such as pagination. `0.9` had
this feature in place.

This adds support for it, as well as fixes small things in README.md.

This won't support `meta` in array responses because arrays don't have
keys, obviously. Also, the response should have a `root` key, otherwise
no `meta` will be included.

In some cases, for example using JsonApi, ArraySerializer will result in
a response with a `root`. In that case, `meta` will be included.

README.md

@@ -54,18 +54,17 @@ end
 ```
 
 Generally speaking, you as a user of AMS will write (or generate) these
-serializer classes. By default, they will use the JsonApiAdapter, implemented
-by AMS. If you want to use a different adapter, such as a HalAdapter, you can
+serializer classes. If you want to use a different adapter, such as a JsonApi, you can
 change this in an initializer:
 
 ```ruby
-ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::HalAdapter
+ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::JsonApi
 ```
 
 or
 
 ```ruby
-ActiveModel::Serializer.config.adapter = :hal
+ActiveModel::Serializer.config.adapter = :json_api
 ```
 
 You won't need to implement an adapter unless you wish to use a new format or
@@ -99,15 +98,6 @@ end
 In this case, Rails will look for a serializer named `PostSerializer`, and if
 it exists, use it to serialize the `Post`.
 
-### Built in Adapters
-
-The `:json_api` adapter will include the associated resources in the `"linked"`
-member when the resource names are included in the `include` option.
-
-```ruby
-  render @posts, include: 'authors,comments'
-```
-
 ### Specify a serializer
 
 If you wish to use a serializer other than the default, you can explicitly pass it to the renderer.
@@ -129,6 +119,46 @@ render json: @posts, each_serializer: PostPreviewSerializer
 render json: @posts, serializer: PaginatedSerializer, each_serializer: PostPreviewSerializer
 ```
 
+### Meta
+
+If you want a `meta` attribute in your response, specify it in the `render`
+call:
+
+```ruby
+render json: @post, meta: { total: 10 }
+```
+
+The key can be customized using `meta_key` option.
+
+```ruby
+render json: @post, meta: { total: 10 }, meta_key: "custom_meta"
+```
+
+`meta` will only be included in your response if there's a root. For instance,
+it won't be included in array responses.
+
+### Root key
+
+If you want to define a custom root for your response, specify it in the `render`
+call:
+
+```ruby
+render json: @post, root: "articles"
+```
+
+### Built in Adapters
+
+#### JSONAPI
+
+This adapter follows the format specified in
+[jsonapi.org/format](http://jsonapi.org/format). It will include the associated
+resources in the `"linked"` member when the resource names are included in the
+`include` option.
+
+```ruby
+  render @posts, include: 'authors,comments'
+```
+
 ## Installation
 
 Add this line to your application's Gemfile: