Remove mention of extras from README
Remove mention of extras from README

Users should install fedmsg through their system package manager, and thus they
should need no instructions on the Python package structure.

vivekanand1101/fedmsg,vivekanand1101/fedmsg,fedora-infra/fedmsg,fedora-infra/fedmsg,maxamillion/fedmsg,vivekanand1101/fedmsg,chaiku/fedmsg,cicku/fedmsg,pombredanne/fedmsg,pombredanne/fedmsg,cicku/fedmsg,cicku/fedmsg,fedora-infra/fedmsg,pombredanne/fedmsg,maxamillion/fedmsg,chaiku/fedmsg,chaiku/fedmsg,maxamillion/fedmsg