Update README.md

Author: thinkingserious

README.md

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 **The default branch name for this repository has been changed to `main` as of 07/27/2020.**
 
+## Announcements
+
+Our Developer Experience team is conducting planned maintenance from 03/09/2021 until 03/11/2021. Our next release is scheduled for 03/15/2021.
+
 ## Documentation
 
 The documentation for the Twilio API can be found [here][apidocs].