link to contributing guidelines in pr template

.github/PULL_REQUEST_TEMPLATE.md

-* Link the PR to an issue for new features
 * Rebase your PR if it gets out of sync with master
-* Follow the contribution guidelines in `CONTRIBUTING.md`
+* Follow the contribution guidelines in [`CONTRIBUTING.md`](https://github.com/grafana/grafana/blob/master/CONTRIBUTING.md)
+* Include `closes #<issue>` or a link to the issue in the description
 
 **REMOVE THE TEXT ABOVE BEFORE CREATING THE PULL REQUEST**