Add “needs-review” to Pull Request template

As suggested by Artem.

Author: ffaf1

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,8 @@
+---
+labels: ["attention: needs-review"]
 
 ---
+
 Please include the following checklist in your PR:
 
 * [ ] Patches conform to the [coding conventions](https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#other-conventions).