Docs: installation instructions (including oidc)

[davidhuser]

Currently there is no basic documentation on actually installing the package and the optionals.

pip install dash-auth

for OIDC support

pip install "dash-auth[oidc]"

Maybe these should be added to the README?

[carosariosb]

This is sooooooo useful, just addding this tag saved me hours of searching. Thanks @davidhuser