Add quickstart description

[ZoomTen]

It's the one for Debian, for now.

Should address #14 even though it's not "quite" a one-liner, since it also covers installing the dependencies.

Some notes while writing the quickstart:

• Should we be able to select stable (latest version) and devel (master) versions?
• Making the curl URL tidier (could be by e.g. making a URL in gbdev.io that redirects to the script)
• Could use a command to print the rgbenv dirs
• Or maybe take inspiration from version managers that have an eval $(version_manager init) command?
• The script can be placed locally, of course, but placing it in /usr/local/bin is just a convenience thing (no need to track additional $PATH dirs—do we need an additional install.sh script?

[ISSOtm]

The placement and general idea look good to me; there are things I'd question about some details, though.

[ISSOtm]

Looks good to me!