[WIP] Document creating extensions

This config fails on my setup. (latest behat)

> [Behat\Testwork\ServiceContainer\Exception\ExtensionInitializationException]
>   `Behat\MinkExtension` extension file or class could not be located.

`$ composer require behat/mink-extension:*` changed the error into

> [Symfony\Component\Config\Definition\Exception\InvalidConfigurationException]
>  The child node "sessions" at path "testwork.mink" must be configured.

It would be nice to have a paragraph about how to resolve this.

Using `$ bin/behat --config-reference` did not help me.