Add PR template for adding a new dataset

[J535D165]

A Pull Request introducing a new dataset should contain the information described in this comment #104 (comment). A PR template with checkboxes can be useful to streamline the creation, review, and management of new PRs.

[EmilyWes]

I did some research, and it turns out you can only use a template when you make a PR to the default branch. As we do the development and adding datasets to another branch, this would not work us.

[J535D165]

Let's add this template to the default branch.

[Rensvandeschoot]

Great idea!!
We should also add to copy the inclusion criteria to the toml file with block quotes.