Update README regarding z-probe type

[michael-preuss]

As I learned here the difference between type 1 and 2 is not just the z-probe offset but also the switch type - NC (normally closed) or NO (normally open).
I broke mine by ramming it into the bed 😄, and I can confirm that it was a type 1, and it was NO, i.e. the contacts were closed when you pushed the switch. Now I replaced the switch and the probe became a type 2 because the microswitch is a shorter NC one, i.e. the contact is interrupted when you push it.
Maybe you could add this to the README.