add pr template

Author: Nadia Eghbal

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,6 @@
+- [ ] Have you followed the [contributing guidelines](https://github.com/github/opensource.guide/blob/gh-pages/CONTRIBUTING.md)?
+- [ ] Have you explained what your changes do, and why they add value to the Guides?
+
+**Please note: we will close your PR without comment if you do not check the boxes above and provide ALL requested information.**
+
+-----