Add tip to point to the official GitHub CLI tool

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.

Author: HonkingGoose

book/06-github/sections/2-contributing.asc

@@ -49,6 +49,13 @@ This is basically the Integration Manager workflow covered in <<ch05-distributed
 
 Let's walk through an example of proposing a change to an open source project hosted on GitHub using this flow.
 
+[TIP]
+====
+You can use the official *GitHub CLI* tool instead of the GitHub web interface for most things.
+The tool can be used on Windows, MacOS, and Linux systems.
+Go to the https://cli.github.com/[GitHub CLI homepage] for installation instructions and the manual.
+====
+
 ===== Creating a Pull Request
 
 Tony is looking for code to run on his Arduino programmable microcontroller and has found a great program file on GitHub at https://github.com/schacon/blink[].