Add tip to point to the official GitHub CLI tool

[HonkingGoose]

• I provide my work under the project license.
• I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Add tip to point to the official GitHub CLI tool

Context

Fixes #1530

The GitHub CLI lets you do your GitHub work from within the command line.
GitHub has just released version 1.0, and so we can "recommend" it to our readers.

I think this is a logical place for the tip. If you want me to move it, let me know. 😉

For more information read the GitHub blog post announcing GitHub CLI 1.0 and go to the GitHub CLI project page.

[ben]

Yes, this is great.

[HonkingGoose]

Changes:

• Rebased so this commit sits on top of current master.
• Link to issue Include GitHub CLI in GITHUB section #1530 so that it's closed when you merge this pull.