requests_oauthlib library already installed

[slootjes]

Not so much as an issue but more to help others: I was just trying HA Docker version 0.98.2 and it turns out the requests_oauthlib library is already installed so no additional work is required to set this up.

I can provide a PR to reflect this in the documentation but I'm not sure if this applies to more versions/distributions.

[kloknibor]

it should be installed by hass itself but for some reason it seemed to fail sometimes... Thats why the comment is there