Improve documentation for adding a new toolhead

[EiNSTeiN-]

I took some notes while installing my new extruder and cleaned them up here: https://gist.github.com/EiNSTeiN-/0035b747aac1895fe47b7ff0676973ad. I'll keep working on them but hopefully this is useful to someone, since the instructions in setup.md are a bit light on details...

I'm opening an issue instead of a pull request because I thought it would be useful to get some eyes on these instructions, I don't know if everything is correct or the easiest way to do everything. I've been following these steps to setup my own extruders successfully so I thought I'd share them to get some feedback.

[viesturz]

Thanks, this is nice. Will adapt what I can. The entire project has moved away from the rails however.