Github CLI

## Overview

Add **Github CLI** content;

**Github CLI** is a great tool and we should add it to the guide.

### Extra detail

First add in the **Install** section, and as **Next Steps**, add tool content.

#### Reasoning

- [ ] Add to the install section
- [ ] Add content about...
- [ ] Create repos
- [ ] Open issues and pull requests, etc

### How to contribute

- Comment on this issue if you'd like to work/collaborate on it!
- Fork the [repository](https://github.com/github/training-kit).
- Use the [GitHub Flow](https://guides.github.com/introduction/flow/) to make changes to your fork. [This](https://github.com/skills/introduction-to-github) is a refresher course if you're unsure about how to make a change on GitHub.
- Push your changes to your repository.
- Submit a Pull Request
- Base Dropdown: github/training-kit
- Compare Dropdown: Your fork

#### Testing locally

If you'd like to make and test changes locally (and see how they would look if merged), do the following:

- In your command line:
- [Install Git](https://git-scm.com/)
- [Install Ruby](https://www.ruby-lang.org/en/documentation/installation/)
- [ ] Run `script/setup`
- [ ] Run `script/server`.
- When successful, the script will initiate a local server at `http://127.0.0.1:4000/`.

### Questions?

- Leave a comment on this issue! Make sure to use `@mentions` if you want a specific person's attention!