Add engine state diagram

Add a nice visual representation of the states the action
engine yields back while executing (which are available from
its run_iter method) and give a brief overview of what these
states mean and how they are used (in diagram and in text).

Change-Id: Idc6abc83e0ed7510c0e4c58b7fcfc26cce3e98a2
Merge "Add engine state diagram"