fix(contributing): add conventional commits link

BD103 · web-flow · commit 85cd35d163a1 · 2021-05-26T08:28:13.000-04:00
diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
@@ -20,4 +20,4 @@ $ isort .
 $ black .
 ```
 
-Then create your PR. Thanks for helping!
+Then create your PR. All commits should follow [Conventional Commits](https://www.conventionalcommits.org). (Look at the commit history for examples.) Thanks for helping!
