Added information on plugin mechanism to glossary

Added extra information regarding the plugin mechanism for:
action, strategy, and Watcher planner.

Change-Id: I9a7523282e229b83c16b06e3806ff795a0699c78
Closes-Bug: #1558470

watcher/decision_engine/strategy/strategies/base.py

@@ -30,6 +30,10 @@ to find an optimal :ref:`Solution <solution_definition>`.
 When a new :ref:`Goal <goal_definition>` is added to the Watcher configuration,
 at least one default associated :ref:`Strategy <strategy_definition>` should be
 provided as well.
+
+:ref:`Some default implementations are provided <watcher_strategies>`, but it
+is possible to :ref:`develop new implementations <implement_strategy_plugin>`
+which are dynamically loaded by Watcher at launch time.
 """
 
 import abc