Added PR template (#2959)

* Added PR template

* Introduction wording improvement

* Update PULL_REQUEST_TEMPLATE.md

Author: lucaswoj

PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,13 @@
+<!--
+Hello! Thanks for contributing. To help your PR be most easily reviewed, please complete
+the following checklist:
+-->
+
+## Checklist
+
+ - [ ] briefly describe the changes in this PR
+ - [ ] write tests for all new functionality
+ - [ ] [document any changes or additions to public APIs](https://github.com/mapbox/mapbox-gl-js/blob/master/docs/README.md)
+ - [ ] [post scores for all benchmarks on your branch and `master`](https://github.com/mapbox/mapbox-gl-js/blob/master/bench/README.md#running-benchmarks)
+ - [ ] [do a quick sanity check on the debug page](https://github.com/mapbox/mapbox-gl-js/blob/master/CONTRIBUTING.md#serving-the-debug-page)
+ - [ ] get a PR review :+1: / :-1: