Create grape integration documentation.

Author: xn

docs/howto/grape_integration.md

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+The AMS grape formatter relies on the existence of `env['grape.request']` which is implemeted by `Grape::Middleware::Globals`. You can meet his dependency by calling it before mounting the endpoints.
+
+In the simpliest way:
+
+```
+class API < Grape::API
+  # @note Make sure this is above you're first +mount+
+  use Grape::Middleware::Globals
+end
+```
+
+or more like what is shown in current Grape tutorials:
+
+```
+module MyApi
+  class ApiBase < Grape::API
+    use Grape::Middleware::Globals
+    
+    require 'grape/active_model_serializers'
+    include Grape::ActiveModelSerializers
+    
+    mount MyApi::V1::ApiBase
+  end
+end
+```
+
+You could meet this dependency with your own middleware. The invocation might look like:
+
+```
+module MyApi
+  class ApiBase < Grape::API
+    use My::Middleware::Thingamabob
+    
+    require 'grape/active_model_serializers'
+    include Grape::ActiveModelSerializers
+    
+    mount MyApi::V1::ApiBase
+  end
+end
+```