Update pull request template (#825)

* update pr template

* Update formatting

* Use checkboxes

Co-authored-by: Richie Bendall <richiebendall@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,3 +5,16 @@ Please read and follow these instructions before creating and submitting a pull
 - Before adding a feature, it is best to create an issue explaining it first. It would save you some effort in case we don't consider it should be included in node-fetch.
 - If you are reporting a bug, adding failing units tests can be a good idea.
 -->
+
+**What is the purpose of this pull request?**
+
+- [ ] Documentation update
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Other, please explain:
+
+**What changes did you make? (provide an overview)**
+
+**Which issue (if any) does this pull request address?**
+
+**Is there anything you'd like reviewers to know?**