add __init__ header to README

Author: cadenmyers13

README.rst

@@ -63,6 +63,23 @@ obtain the total cost function. Additionally, diffpy.srfit is designed to be
 extensible, allowing the user to integrate external calculators to perform
 co-refinements with other techniques.
 
+
+SrFit has tools for coherently combining known information about a
+material to derive other properties, in particular material structure.
+SrFit allows the customization and creation of structure
+representations, profile calculators, constraints, restraints and file
+input parsers. The customized pieces can be glued together within SrFit
+to optimize a structure, or other physically relevant information from
+one or more experimental profiles. Other known information about the
+system of interest can be included with arbitrarily complex constraints
+and restraints. In this way, the end user creates a customized fitting
+application that suits the problem to the available information.
+
+The subpackages herein define various pieces of the SrFit framework.
+Developers are encouraged to work through the examples described in the
+documentation to learn how to use and customize the various parts of
+SrFit.
+
 For more information about the diffpy.srfit library, please consult our `online documentation <https://diffpy.github.io/diffpy.srfit>`_.
 
 Citation