Add a skeleton of the guide to ease user contributions

Has some stuff that aren't in master yet, and lacks a lot more
that is already in master.

guide/loop_handlers.md

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+Loop handlers
+=============
+
+Purpose
+-------
+
+Loop handlers are a special kind of HTTP handlers used when the
+response can not be sent right away. The handler enters instead
+a receive loop waiting for the right message before it can send
+a response.
+
+They are most useful when performing long-polling operations or
+when using server-sent events.
+
+Callbacks
+---------
+
+@todo Describe the callbacks.
+
+Usage
+-----
+
+@todo Explain how to use them.