Documentation on goal and efficacy

In this changeset, I wrote a documentation detailing how one can
implement a new goal plugin. I also mention to define the efficacy
specification for a given goal.

Partially Implements: blueprint efficacy-indicator

Change-Id: Iba267ae312f248b49d4600504f11678cdc225622

watcher/decision_engine/solution/base.py

@@ -18,10 +18,17 @@
 #
 
 """
-A :ref:`Solution <solution_definition>` is a set of
-:ref:`Actions <action_definition>` generated by a
-:ref:`Strategy <strategy_definition>` (i.e., an algorithm) in order to achieve
-the :ref:`Goal <goal_definition>` of an :ref:`Audit <audit_definition>`.
+A :ref:`Solution <solution_definition>` is the result of execution of a
+:ref:`strategy <strategy_definition>` (i.e., an algorithm).
+Each solution is composed of many pieces of information:
+
+- A set of :ref:`actions <action_definition>` generated by the strategy in
+  order to achieve the :ref:`goal <goal_definition>` of an associated
+  :ref:`audit <audit_definition>`.
+- A set of :ref:`efficacy indicators <efficacy_indicator_definition>` as
+  defined by the associated goal
+- A :ref:`global efficacy <efficacy_definition>` which is computed by the
+  associated goal using the aforementioned efficacy indicators.
 
 A :ref:`Solution <solution_definition>` is different from an
 :ref:`Action Plan <action_plan_definition>` because it contains the