PR template for github

Author: adrn

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,6 @@
+- [ ] Please check out our [contributing
+  guidelines](https://learn.astropy.org/contributing/how-to-contribute) and/or
+  indicate (check the box) that you are familiar with our contributing workflow.
+- [ ] Please also be sure to check out our [code of
+  conduct](https://github.com/astropy/astropy/blob/main/CODE_OF_CONDUCT.md) and
+  check this box to agree to follow the CoC.