Update config to accept secrets stored in environmental variables.

[tavsec]

I updated the default github.php config so it accepts GitHub credentials that can be stored in environmental variables, so they do not have to be hard-coded in the configuration file.

I also updated the README.md accordingly.

[GrahamCampbell]

I know some packages do it, but I hate to force people into having env variables read by default when they weren't expecting it. I'd far rather people publish the config and then adjust it as they need. In particular, the names of the connections are just examples, and are not necessarily meant to be kept as is.