pydispatch docs: added minimalistic intro (from Vaclav email 2013)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@62359 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/python/pydispatch/__init__.py

@@ -1,6 +1,25 @@
 """Multi-consumer multi-producer dispatching mechanism
-"""
-__version__ = "2.0.3"
-__author__ = "Patrick K. O'Brien"
-__license__ = "BSD-style, see license.txt for details"
 
+This Python library is used e.g. in wxGUI. The purpose of the library
+is to provide mechanism for communication between objects in wxGUI.
+The library consists of two parts:
+
+* the Python Signal API which will be used in wxGUI, and
+* this 3rd party package PyDispatcher does the hard work.
+
+In short, simple function calls are not sufficient in the GUI, event
+driven and large environment with many persistent objects because
+using simple function calls would lead to tightly coupled code. Thus,
+some better mechanism is needed such as Observer design pattern. In
+GRASS GIS, we use the Signal system which is similar to Signals used in
+PyQt and other frameworks. As the underlying library, we have chosen
+PyDispatcher because it provides very general API which enables to
+implement Signal API, wide and robust functionality which makes
+implementation and use of Signals easier.
+
+PyDispatcher metadata:
+
+:version: 2.0.3
+:author: Patrick K. O'Brien
+:license: BSD-style, see license.txt for details
+"""