Chore: update README.md with MVR notice

Author: JenniferMah

README.md

@@ -5,7 +5,8 @@
 [![PyPI](https://img.shields.io/pypi/pyversions/twilio.svg)](https://pypi.python.org/pypi/twilio)
 [![Learn OSS Contribution in TwilioQuest](https://img.shields.io/static/v1?label=TwilioQuest&message=Learn%20to%20contribute%21&color=F22F46&labelColor=1f243c&style=flat-square&logo=data:image/png;base64,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)](https://twil.io/learn-open-source)
 
-**The default branch name for this repository has been changed to `main` as of 07/27/2020.**
+* **The default branch name for this repository has been changed to `main` as of 07/27/2020.**
+* **The upcoming release of the Twilio Python Helper Library (7.0.0) on September 22, 2021 will not be compatible with Python versions 2.7, 3.4, and 3.5. Projects which use Python versions 2.7, 3.4, 3.5 or earlier will not be able to upgrade the Twilio Helper Library from 7.0.0 onwards.**
 
 ## Documentation