Document what paralell feature vs parallel scenario means exactly

[jessedoj]

This references parallel feature and parallel scenario multiple times but never exactly delineates what they are and what the difference is. Add a short little explanation to the readme file relating to this.

[Daniel-Marynicz]

Parallel scenario fires scenario tests as separate threads. Parallel feature fires feature tests as separate threads. What is a scenario and feature can be found at https://behat.org/en/latest/user_guide/writing_scenarios.html https://behat.org/en/latest/user_guide/organizing.html

[jessedoj]

Great, like it sounds. Let's add that to the readme!