Added docstring to public class

Author: mmegs-amzn

src/aws_encryption_sdk/keyring/base.py

@@ -14,6 +14,8 @@
 
 
 class Keyring(object):
+    """Parent interface for Keyring classes.
+    """
     def on_encrypt(self, encryption_materials):
         """Generates a data key and encrypts it using all wrapping keys the Keyring is associated with.