add pull request template

.github/PULL_REQUEST_TEMPLATE.md

+## What does this PR do?
+Add Resource(s) | Remove Resource(s) | Add info | Improve Repo
+
+## For resources
+### Description 
+
+### Why is this valuable (or not)
+
+### How do we know it's really free?
+
+### For book lists, is it a book?
+
+### Checklist:
+- [ ] Not a duplicate
+- [ ] Included author(s) if appropriate
+- [ ] Lists are in alphabetical order
+- [ ] Needed indications added (PDF, access notes, under construction)