[1.8.x] Updated contributing link in the README.
[1.8.x] Updated contributing link in the README.

Backport of f7bf135f62297c580e9b36af4c2470adb652fbaf from master

pquentin/django,h4r5h1t/django-hauthy,pquentin/django,pquentin/django,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy