Update readme; Switch default branch to main

Author: techgique

README.md

@@ -5,7 +5,7 @@ and templates for finding content for a given date.
 
 ## Compatibility
 
-The "master" branch should not be considered stable.  Unlike the core Open ONI
+The "main" branch should not be considered stable.  Unlike the core Open ONI
 repository, plugins don't warrant the extra overhead of having separate
 development branches, release branches, etc.  Instead, it is best to find a tag
 that works and stick with that tag.