Document support for GITHUB_TOKEN

[ocean90]

#47 added support for the GITHUB_TOKEN and COMPOSER_AUTH env. But it's not documented.

I think we should mention the support on the wp package install command so it's visible on https://developer.wordpress.org/cli/commands/package/install/.

[spicecadet]

Just a heads up that I worked on documentation for this during WCEU contributor day in Athens. Let me know if there's anything else needed here.