Readme: Add note for eventlet installtion
    
    We use /usr/bin/celery for testing which uses
    eventlet outside of venv. Thus, in order to run
    the tests successfully, we need to install eventlet
    outside of ven
    
    Signed-off-by: Vivek Anand <vivekanand1101@gmail.com></vivekanand1101@gmail.com>