Nteract: Create a PR template

[rgbkrk]

Since we have some of our commit guidance buried away in a guide, we should create a PR template that GitHub can pick up. I'm guessing we could make it similar to redux-observable's:

<!-- If this is your first PR for nteract, please mark these boxes to confirm (otherwise you can exclude them)-->

- [ ] I have read the [Contributor Guide](https://github.com/nteract/nteract/blob/master/CONTRIBUTING.md)
- [ ] My commit messages are in [conventional-changelog-standard](https://github.com/nteract/nteract/blob/master/CONTRIBUTING.md#how-should-i-write-my-commit-messages-and-pr-titles) format.

Open to opinions here, might as well start with something.

[jdetle]

Thought: We could include Questions? Feel free to ping us on [slack](https://slack.nteract.in)

[LBKinson]

I was unsure of PR title verbage, so I would encourage your second bullet to be:
- [ ] My PR title and commit messages are in [conventional-changelog-standard](https://github.com/nteract/nteract/blob/master/CONTRIBUTING.md#how-should-i-write-my-commit-messages-and-pr-titles) format.

[rgbkrk]

I was unsure of PR title verbage... My PR title and commit messages...

Whoa! Totally. Glad you noticed.

[lgeiger]

This was closed in #1082
Thanks to @vperezma 👍

[lock[bot]]

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