added intro to readme

README.rst

@@ -9,3 +9,10 @@ Installation
 ::
 
   pip install manipulator
+
+Usage
+-----
+
+`manipulator` mainly exposes three functions, `get`, `update`, and `set`.
+`get` retrieves data, `update` transforms it based on its form, and `set`
+transforms it by simply resetting.