[PULL_REQUEST_TEMPLATE.md] Add explicit entry on flake8

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,6 +9,7 @@
 ### Before submitting a *pull request* make sure you have:
 - [ ] At least skimmed through [adding new extractor tutorial](https://github.com/rg3/youtube-dl#adding-support-for-a-new-site) and [youtube-dl coding conventions](https://github.com/rg3/youtube-dl#youtube-dl-coding-conventions) sections
 - [ ] [Searched](https://github.com/rg3/youtube-dl/search?q=is%3Apr&type=Issues) the bugtracker for similar pull requests
+- [ ] Checked the code with [flake8](https://pypi.python.org/pypi/flake8)
 
 ### In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under [Unlicense](http://unlicense.org/). Check one of the following options:
 - [ ] I am the original author of this code and I am willing to release it under [Unlicense](http://unlicense.org/)