Add a requirements.txt to scripts folder

Some scripts require external modules (from pip).
Install using:

 python -m pip install -r requirements.txt

NOTE: On some distributions python2 is still default, so to
use python 3 it might be necessary to replace python on the
above command line with python3

Update README.md to mention the python requirements.

Change-Id: Ia7581c874570d92846095775f680238328f651b4