Edit CONTRIBUTING.md

Added a missing comma

Author: sclaire-1

.github/CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 ## Quality Standard
 
-To ensure the continued stability of this package tests are required to be written or already exist in order for a pull request to be merged.
+To ensure the continued stability of this package, tests are required to be written or already exist in order for a pull request to be merged.
 
 ## Reporting issues