Implement a PR template with style guide check list

[pllim]

It would be beneficial in the long run to have a PR template such that when someone opens a new PR in this repo to submit a notebook, one would see a check list of standards that need to be met for the notebook to be acceptable. Reviewer can tick off the check list as appropriate in the review process. What do you think, @eteq ?

p.s. When this process is mature enough to be automated, we might even be able to unleash a review bot to do the first pass... 🤖