Adding warning in example JOs, uising main31 except for bb4l, adding more infos in README
(cherry picked from commit da0b19c0)
The PowhegControl readme is updated to add more infos on the Pythia8 setting. The example shown there now uses the main31 routines. The example joboptions were also updated to use the main31 routine (except bblvlv where special settings are needed) and the following warning was added:
#--------------------------------------------------------------
# This is an example joboption to generate events with Powheg
# using ATLAS' interface. Users should optimise and carefully
# validate the settings before making an official sample request.
#--------------------------------------------------------------