Add sentinel auth to an exemple in the documentation.

[Christophe31]

Problem Statement
django-redis seems to be the best way to add sentinel feature of redis to django cache… the authentication is unclear to add and seems undocumented.

Describe the solution you'd like
either use the "PASSWORD" for sentinel connection, or add authenticated sentinel params to the README

ex: #610

[WisdomPill]

thanks @Christophe31 for raising the issue? would you like to open a PR to fix the issue?

[Pefington]

Hi, I struggled a bit to set it up.
I made a gist of a config that works.
https://gist.github.com/Pefington/c60cd5f9c99cb960fe2a09d2855e5eac

If you confirm my understanding is right, I can try and open a quick PR on the documentation.