Add Pull Request Template

[parsonsmatt]

This PR adds a template for Pull Requests.

[Lysxia]

@Bodigrim Is there a policy for contributors to know upfront what the next version number is going to be? That seems a blocking thing to make the second item actionable. Should it be X.Y.(Z+1).0 always, so by default we merge only PVP-minor-level changes, and breaking changes get merged once in a blue moon?

[Bodigrim]

@Lysxia to be honest, my experience of burdening contributors with admin tasks is unsatisfactory. A new function could be targetting a major release, but end up being backported to a stable branch, etc. The practice of bytestring and vector is that a release manager goes through changes and adds @since pragmas where appropriate.

[Boarders]

I agree with @Bodigrim, I also personally found it quite confusing when I was trying to add to a library and this issue came up.