Update README to include testing section

Add a testing section to the README.

Author: arandito

README.rst

@@ -11,3 +11,5 @@ This project is maintained and published by Amazon Web Services.
   production, make sure to lock to a minor version as interfaces may break
   from minor version to minor version. For a basic, stable interface of
   s3transfer, try the interfaces exposed in `boto3 <https://docs.aws.amazon.com/boto3/latest/guide/s3.html>`__
+
+testing