Draft PR contributor guide link language

[rbs-jacob]

• [x] I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!)

Add language to the PR template about the OFRAK contributor guide.

Happy to change the language to whatever people think is best, but having a link to the contributor guide front-and-center would be helpful.

Anyone you think should look at this, specifically? @whyitfor