add punctuation

CONTRIBUTING.md

@@ -18,7 +18,7 @@ Before you open a PR:
   know. We'll help you get started, rather than inadvertently doubling up on your hard work.
 - Make sure you do not add regressions by running `yarn test`.
 - Where possible, include tests with your changes, either that demonstrates the
-  bug, or tests the new functionality
-- Please [follow our contributors guidelines]([https://github.com/keystonejs/keystone/wiki/Coding-Standard](https://github.com/JedWatson/react-select/blob/master/CONTRIBUTING.md))
+  bug, or tests the new functionality.
+- Please [follow our contributors guidelines]([https://github.com/keystonejs/keystone/wiki/Coding-Standard](https://github.com/JedWatson/react-select/blob/master/CONTRIBUTING.md)).
   (with regards to formatting, etc)
 - All new features and changes need documentation.