Add PR checklist.

[HarshKapadia2]

Add PR checklist primarily to remind people to run Prettier before opening a PR.

Credits: @KartikSoneji

PS: Which label should one apply to such an issue? Do we need a new label? If yes, what would it be called?

[KartikSoneji]

I was actually thinking of making it an optional pre-commit hook that people can enable if they choose to.

[KartikSoneji]

Oh just saw #126.
Closing this in favour of that.

[HarshKapadia2]

Wait why not have both, an optional pre-commit hook and a PR checklist template, @KartikSoneji?

The pre-commit hook gives an additional way to use Prettier and the checklist serves as a reminder if one hasn't used Prettier...