DOC: Add description of package to README.rst

Author: zbrookle

README.rst

@@ -19,6 +19,12 @@ dataframe_sql
 .. image:: https://img.shields.io/badge/code%20style-black-000000.svg
     :target: https://github.com/psf/black
 
+``dataframe_sql`` is a Python_ package that translates SQL syntax into operations on
+pandas_ DataFrames, a functionality which is not available in the central pandas
+package.
+
+.. _Python: https://www.python.org/
+
 Installation
 ------------