Add base_url to docs for configuring Mink

[liquorvicar]

Fixes #95

[pamil]

The base of this pull-request was changed, you need fetch and reset your local branch
if you want to add new commits to this pull request. Reset before you pull, else commits
may become messed-up.

Unless you added new commits (to this branch) locally that you did not push yet,
execute git fetch origin && git reset "add-base-url-to-docs" to update your local branch.

Feel free to ask for assistance when you get stuck 👍

[pamil]

Thank you, Andy! 🥇