chore(github): Add "Closes" to PR template

Author: s1gr1d

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,3 +3,6 @@ Before submitting a pull request, please take a look at our
 
 - [ ] If you've added code that should be tested, please add tests.
 - [ ] Ensure your code lints and the test suite passes (`yarn lint`) & (`yarn test`).
+- [ ] Link an issue if there is one related to your pull request. If no issue is linked, one will be auto-generated and linked.
+
+Closes #issue_link_here