Add a note about GitHub CLI

Avoid literal mention of "GitHub CLI" because hub already ranks too high
for those search terms and we don't want it to rank any higher.

Closes mislav#2458

Author: mislav

README.md

@@ -1,6 +1,10 @@
 hub is a command line tool that wraps `git` in order to extend it with extra
 features and commands that make working with GitHub easier.
 
+For an official, potentially more user-friendly command-line interface to GitHub,
+see [cli.github.com](https://cli.github.com) and
+[this comparison](https://github.com/cli/cli/blob/trunk/docs/gh-vs-hub.md).
+
 This repository and its issue tracker is **not for reporting problems with
 GitHub.com** web interface. If you have a problem with GitHub itself, please
 [contact Support](https://github.com/contact).