doc: add Muter to the example list

Author: nkcsgexi

README.md

@@ -89,6 +89,8 @@ the Swift compiler source because we always push gyb-generated files to a tag.
 
 [**BartyCrouch**](https://github.com/Flinesoft/BartyCrouch): a tool to incrementally update strings files to help App localization.
 
+[**Muter**](https://github.com/SeanROlszewski/muter): Automated mutation testing for Swift
+
 ## Contributing
 
 ### Building SwiftSyntax from `master`