Update docs with OpenRC Information

[markhughes]

Although operating systems like Gentoo and Alpine Linux use a 'initd' they use a slight alternative known as OpenRC.

So this won't work:

$ sudo update-rc.d minecraft defaults
$ sudo update-rc.d minecraft enable

Instead, the following should be added to the docs for OpenRC:

$ rc-update add minecraft

[AdorablePotato]

Can you create a pull request for this? I'm not familiar with OpenRC.