Getting started: "Installing from source" only applies to <=0.13
Getting started: Mention the irNetBox

It's already mentioned in introduction.html and in stbt.html (the man
page) but it doesn't hurt to mention it here too.
Getting started: Add wiring diagram

There was already a similar diagram in jenkins.html, but not everybody
will look there. It makes more sense to have a diagram here, in "Getting
started". Unlike the jenkins diagram, this diagram includes the IR
receiver needing for recording tests.