add "additional information" section, and link to guidelines

Author: emilykl

.github/pull_request_template.md

@@ -19,3 +19,11 @@ Closes #(issue number)
 ## Testing strategy
 
 <!-- Provide 1-2 sentences explaining tests added or changed by this PR. If testing changes are not needed, explain why. -->
+
+## Additional information (optional)
+
+<!-- Include any additional context, background, or explanation which doesn't fit in the previous sections. -->
+
+## Guidelines
+
+- [ ] I have reviewed the [pull request guidelines](https://github.com/plotly/plotly.py/blob/update-pr-template/CONTRIBUTING.md#opening-a-pull-request) and confirmed that this PR follows them.