Add pull request template

[delucchi-cmu]

Add a pull request template for changes to the python-project-template repo.

Specifically, add some reminder that this is creating a pull request in the lincc-frameworks/python-project-template repository.

Now that we have several supported forks of the repo, it's a little too easy to accidentally create a pull request against the wrong repo, and github has no plans to set a default repo for pull requests for forked repositories.

[drewoldag]

Should add some other checkboxes for the PR template. i.e. does this change require some new CI tests, etc.