Update pull_request_template.md

Added checkbox for flake8 linter

Author: hlflanagan

pull_request_template.md

@@ -5,6 +5,6 @@
 * [ ] Have you added information on what your changes do and why you chose this as your solution?
 * [ ] Have you written new tests for your changes?
 * [ ] Does your submission pass tests?
-* [ ] Have you lint your code locally prior to submission?
+* [ ] This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?