Add a reminder to use underscores in file and directory names to the PR checklist (#2246)

Author: seisman

.github/PULL_REQUEST_TEMPLATE.md

@@ -16,6 +16,7 @@ Fixes #
 - [ ] Write detailed docstrings for all functions/methods.
 - [ ] If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
 - [ ] If adding new functionality, add an example to docstrings or tutorials.
+- [ ] Use underscores (not hyphens) in names of Python files and directories.
 
 **Slash Commands**