Contributing Guidelines and Pull Request Process

1st chunk of `.github/PULL_REQUEST_TEMPLATE.md`

7b03154bef2b575d9f2e3c810ea1076eb4d209261b407ecc00000001000004da

<!--

IMPORTANT

Nix is a non-trivial project, so for your contribution to be successful,
it really is important to follow the contributing guidelines:

https://github.com/NixOS/nix/blob/master/CONTRIBUTING.md

Even if you've contributed to open source before, take a moment to read it,
so you understand the process and the expectations.

- what information to include in commit messages
- proper attribution
- volunteering contributions effectively
- how to get help and our review process.

-->

## Motivation

<!-- Briefly explain what the change is about and why it is desirable. -->

## Context

<!-- Provide context. Reference open issues if available. -->

<!-- Non-trivial change: Briefly outline the implementation strategy. -->

<!-- Invasive change: Discuss alternative designs or approaches you considered. -->

<!-- Large change: Provide instructions to reviewers how to read the diff. -->

---

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).

Title: Contributing Guidelines and Pull Request Process

Summary

This section emphasizes the importance of following the contributing guidelines for the Nix project, including commit message conventions, attribution, and the review process. It also explains how to signal important pull requests using :+1: and how the Nix maintainer team uses a GitHub project board to schedule and track reviews.