Added documentation for send command

[rhodso]

Hi there!

I run a minecraft server with linuxgsm and I wanted to let my players know that the server would restart 5 mins before it actually would. I searched the docs and there wasn't anything in there that would allow me to do that, so I was going to open an issue in the main repository. After searching through closed issues I found some references to a send command, that seemed undocumented.

I thought it best to write some barebones documentation for the command so that people at least know that it exists. I welcome any feedback that you have about the documentation I've written, but hopefully someone with better knowledge of linuxgsm can make this barebones documentation better,