docs: add link to project board to PRs (#9630)

* docs: add link to project board to PRs

* Update .github/PULL_REQUEST_TEMPLATE.md

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

* fix wording

* add note on the process

---------

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

Author: tomberek

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,6 +10,8 @@
 
 <!-- Large change: Provide instructions to reviewers how to read the diff. -->
 
-# Priorities
+# Priorities and Process
 
 Add :+1: to [pull requests you find important](https://github.com/NixOS/nix/pulls?q=is%3Aopen+sort%3Areactions-%2B1-desc).
+
+The Nix maintainer team uses a [GitHub project board](https://github.com/orgs/NixOS/projects/19) to [schedule and track reviews](https://github.com/NixOS/nix/tree/master/maintainers#project-board-protocol).