doc: document pre-commit for flake8 in HACKING
doc: document pre-commit for flake8 in HACKING

oorestisime/debsources,zacchiro/debsources,oorestisime/debsources,devoxel/debsources,sophiejjj/debsources,vivekanand1101/debsources,sophiejjj/debsources,devoxel/debsources,Debian/debsources,devoxel/debsources,Debian/debsources,matthieucan/debsources,Debian/debsources,matthieucan/debsources,matthieucan/debsources,clemux/debsources,matthieucan/debsources,devoxel/debsources,vivekanand1101/debsources,zacchiro/debsources,clemux/debsources,oorestisime/debsources,sophiejjj/debsources,sophiejjj/debsources,nonas/debian-qa,zacchiro/debsources,vivekanand1101/debsources,nonas/debian-qa,clemux/debsources,vivekanand1101/debsources,zacchiro/debsources,Debian/debsources,devoxel/debsources,nonas/debian-qa,clemux/debsources,oorestisime/debsources,clemux/debsources,matthieucan/debsources,vivekanand1101/debsources,sophiejjj/debsources,nonas/debian-qa,oorestisime/debsources,Debian/debsources,zacchiro/debsources