chore: Add security issue notification to readme

Author: seebees

README.rst

@@ -34,6 +34,8 @@ full documents in our `primary documents`_.
 
 You can find our source on `GitHub`_.
 
+`Security issue notifications`_
+
 ***************
 Getting Started
 ***************
@@ -188,3 +190,4 @@ of the one that the client would normally construct for you.
 .. _CryptoConfig: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/config.html
 .. _decrypt_dynamodb_item: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/item.html#dynamodb_encryption_sdk.encrypted.item.decrypt_dynamodb_item
 .. _transformation functions: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/tools/transform.html
+.. _Security issue notifications: https://github.com/aws/aws-dynamodb-encryption-python/tree/master/CONTRIBUTING.md#security-issue-notifications