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/goal/efficacy/base.py

@@ -14,6 +14,15 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+"""
+An efficacy specfication is a contract that is associated to each :ref:`Goal
+<goal_definition>` that defines the various :ref:`efficacy indicators
+<efficacy_indicator_definition>` a strategy achieving the associated goal
+should provide within its :ref:`solution <solution_definition>`. Indeed, each
+solution proposed by a strategy will be validated against this contract before
+calculating its :ref:`global efficacy <efficacy_definition>`.
+"""
+
 import abc
 import json