chore: update README to reflect default branch rename

README.md

@@ -4,6 +4,8 @@
 [![Travis Build Status](https://travis-ci.org/twilio/twilio-cli.svg?branch=master)](https://travis-ci.org/twilio/twilio-cli.svg?branch=master)[![Appveyor Build Status](https://ci.appveyor.com/api/projects/status/48hf89rslhjhn7ca?svg=true)](https://ci.appveyor.com/project/TwilioAPI/twilio-cli)[![codecov](https://codecov.io/gh/twilio/twilio-cli/branch/master/graph/badge.svg)](https://codecov.io/gh/twilio/twilio-cli)
 [![Learn with 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,iVBORw0KGgoAAAANSUhEUgAAAIAAAACACAMAAAD04JH5AAAASFBMVEUAAAAZGRkcHBwjIyMoKCgAAABgYGBoaGiAgICMjIyzs7PJycnMzMzNzc3UoBfd3d3m5ubqrhfrMEDu7u739/f4vSb/3AD///9tbdyEAAAABXRSTlMAAAAAAMJrBrEAAAKoSURBVHgB7ZrRcuI6EESdyxXGYoNFvMD//+l2bSszRgyUYpFAsXOeiJGmj4NkuWx1Qeh+Ekl9DgEXOBwOx+Px5xyQhDykfgq4wG63MxxaR4ddIkg6Ul3g84vCIcjPBA5gmUMeXESrlukuoK33+33uID8TWeLAdOWsKpJYzwVMB7bOzYSGOciyUlXSn0/ABXTosJ1M1SbypZ4O4MbZuIDMU02PMbauhhHMHXbmebmALIiEbbbbbUrpF1gwE9kFfRNAJaP+FQEXCCTGyJ4ngDrjOFo3jEL5JdqjF/pueR4cCeCGgAtwmuRS6gDwaRiGvu+DMFwSBLTE3+jF8JyuV1okPZ+AC4hDFhCHyHQjdjPHUKFDlHSJkHQXMB3KpSwXNGJPcwwTdZiXlRN0gSp0zpWxNtM0beYE0nRH6QIbO7rawwXaBYz0j78gxjokDuv12gVeUuBD0MDi0OQCLvDaAho4juP1Q/jkAncXqIcCfd+7gAu4QLMACCLxpRsSuQh0igu0C9Svhi7weAGZg50L3IE3cai4IfkNZAC8dfdhsUD3CgKBVC9JE5ABAFzg4QL/taYPAAWrHdYcgfLaIgAXWJ7OV38n1LEF8tt2TH29E+QAoDoO5Ve/LtCQDmKM9kPbvCEBApK+IXzbcSJ0cIGF6e8gpcRhUDogWZ8JnaWjPXc/fNnBBUKRngiHgTUSivSzDRDgHZQOLvBQgf8rRt+VdBUUhwkU6VpJ+xcOwQUqZr+mR0kvBUgv6cB4+37hQAkXqE8PwGisGhJtN4xAHMzrsgvI7rccXqSvKh6jltGlrOHA3Xk1At3LC4QiPdX9/0ndHpGVvTjR4bZA1ypAKgVcwE5vx74ulwIugDt8e/X7JgfkucBMIAr26ndnB4UCLnDOqvteQsHlgX9N4A+c4cW3DXSPbwAAAABJRU5ErkJggg==)](https://twil.io/learn-open-source)
 
+**The default branch name for this repository has been changed to `main` as of 07/27/2020.**
+
 ## Setup
 
 Head over to the [Twilio CLI documentation](https://www.twilio.com/docs/twilio-cli/quickstart).