Update documentation to suggest installing python3-distutils on Ubuntu

[dweekly]

Note that Ubuntu 18 (+ 16 IIRC) don't automatically install the distutils package. A prerequisite for successful build from source is therefore to perform this step before running the setup.py script:

sudo apt-get install python3-distutils

Otherwise users will get this error:

$ python3 setup.py install
Traceback (most recent call last):
File "setup.py", line 6, in
from distutils.core import setup
ModuleNotFoundError: No module named 'distutils.core'

[tomato42]

in CONTRIBUTING we have full list of packages necessary for installation/development, but it's just RHEL 7 and Fedora. Could you propose a list for Ubuntu? Or you're thinking about something different?