Moves documentation link
Moves documentation link

Quickly Googling for this library, and then wanting to navigate to the docs is slightly annoying as the link to the docs is hidden halfway through the main README.rst (after links to other doc sites, like the postgreSQL docs).

This PR adds an earlier reference to the doc site.
tomturner/django-tenants,tomturner/django-tenants,tomturner/django-tenants