BLD: Enforce what's new entry

Ensure PRs add what's new entry

Author: bashtage

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,4 @@
 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
+- [ ] added entry to docs/source/whatsnew/vLATEST.txt