Update README.md (#868)

Adds an explanation of the terminology changes we have made.

Author: catehstn

README.md

@@ -13,6 +13,10 @@ We use git submodules and so when you are checking out the app, you'll need to e
 Alternatively, if you already have the project checked out, you can initialize the submodules manually.
 
     git submodule update --init
+    
+## Terminology
+
+We have taken steps to update our terminology and remove words with problematic racial connotations, most notably the change to `main` branches, `allow lists`, and `blocklists`. Closed issues or PRs may contain deprecated terminology that should not be used going forward.
 
 ## Contribute