Fix: add link in the letter to reviewers template to our conflict of interest statement in the peer review policies

[lwasser]

In our editors guide we have a template letter that editors can send to reviewers in order to invite them to do a review.

Conflict of interest (COI) is important as we want to ensure the reviewers selected identify if they have a COI with the author. As such we should add a statement / link in that template to our conflict of interest text which is here.

https://www.pyopensci.org/software-peer-review/how-to/editors-guide.html