chore: update README to reflect default branch rename

Sam Harrison · Sam Harrison · commit 2b260a62eebc · 2020-07-27T17:11:47.000-05:00
diff --git a/README.rst b/README.rst
@@ -3,6 +3,8 @@
 
 |Build Status| |Email Notifications Badge| |Twitter Follow| |Codecov branch| |Code Climate| |Python Versions| |PyPI Version| |GitHub contributors| |MIT licensed|
 
+**The default branch name for this repository has been changed to `main` as of 07/27/2020.**
+
 **Quickly and easily access any RESTful or RESTful-like API.**
 
 If you are looking for the Twilio SendGrid API client library, please see `this repo`_.
