Add Instructions to use PPK2

This commit includes some instructions on how to use the PPK2, the
instructions are included as a new chapter in the README.md. No text in
other files has been changed or removed. It includes one example that
can be used to check whether the PPK2 works as expected. The example is
a replica of lab4 with an additional PIN turned on and off.

Signed-off-by: Malte Munch <malte.munch@ltu.se>