Update the README and documentation

[mpdude]

This updates the README and improves documentation a bit.

As requested in https://github.com/FriendsOfBehat/MinkExtension/pull/42#pullrequestreview-3592365981, the README is a copy from the original upstream repo with only slight changes.

Closes #27.

[aik099]

@stof , could you please grant me more access to this repo. Right now I'm unable to:

• merge PRs
• approve GitHub Actions execution for new contributor PRs ("1 workflow awaiting approval")
• probably something else

The access level that I have on minkphp organization should be sufficient 😄 .

[stof]

@aik099 I cannot. Only @pamil is owner of this organization.

[mpdude]

If you guys agree I'll migrate to markdown first in a new PR, then later rebase this one and other ones affecting documentation.

[mpdude]

Let's first get #45 merged, then I'll rebase this one here.

[mpdude]

Rebased. Now it's much clearer to see what I intended.