Add initial Ranch guide

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+Embedded mode
+=============
+
+Purpose
+-------
+
+Embedded mode allows you to insert Ranch listeners directly
+in your supervision tree. This allows for greater fault tolerance
+control by permitting the shutdown of a listener due to the
+failure of another part of the application and vice versa.
+
+Embedding
+---------
+
+To embed Ranch in your application you can simply add the child specs
+to your supervision tree. This can all be done in the ```init/1``` function
+of one of your application supervisors.
+
+Ranch requires at the minimum two kinds of child specs for embedding.
+First, you need to add ```ranch_sup``` to your supervision tree, only once,
+regardless of the number of listeners you will use. Then you need to
+add the child specs for each listener.
+
+Ranch has a convenience function for getting the listeners child specs
+called ```ranch:child_spec/6```, that works like ```ranch:start_listener/6```,
+except that it doesn't start anything, it only returns child specs.
+
+As for ```ranch_sup```, the child spec is simple enough to not require a
+convenience function.
+
+The following example adds both ```ranch_sup``` and one listener to another
+application's supervision tree.
+
+``` erlang
+init([]) ->
+    RanchSupSpec = {ranch_sup, {ranch_sup, start_link, []},
+        permanent, 5000, supervisor, [ranch_sup]},
+    ListenerSpec = ranch:child_spec(echo, 100,
+        ranch_tcp, [{port, 5555}],
+        echo_protocol, []
+    ),
+    {ok, {{one_for_one, 10, 10}, [RanchSupSpec, ListenerSpec]}}.
+```
+
+Remember, you can add as many listener child specs as needed, but only
+one ```ranch_sup``` spec!
+
+It is recommended that your architecture makes sure that all listeners
+are restarted if ```ranch_sup``` fails. See the Ranch internals chapter for
+more details on how Ranch does it.