README: advise to use detox for tests
README: advise to use detox for tests

aibon/django-stripe-payments,jawed123/django-stripe-payments,alexhayes/django-stripe-payments,jamespacileo/django-stripe-payments,crehana/django-stripe-payments,adi-li/django-stripe-payments,pinax/django-stripe-payments,boxysean/django-stripe-payments,crehana/django-stripe-payments,jawed123/django-stripe-payments,jamespacileo/django-stripe-payments,ZeevG/django-stripe-payments,aibon/django-stripe-payments,ZeevG/django-stripe-payments,adi-li/django-stripe-payments,boxysean/django-stripe-payments,alexhayes/django-stripe-payments