Link to Mastodon

Author: drbruced12

README.rst

@@ -30,7 +30,7 @@ at `https://ops.systemsapproach.org
 To track progress and receive notices about new versions, you can follow
 the project on
 `Facebook <https://www.facebook.com/Computer-Networks-A-Systems-Approach-110933578952503/>`__
-and `Twitter <https://twitter.com/SystemsAppr>`__. To read a running
+and `Mastodon <https://discuss.systems/@SystemsAppr>`__. To read a running
 commentary on how the Internet is evolving, you can follow the 
 `Systems Approach on Substack <https://systemsapproach.substack.com/>`__.