add "pip install molecule-vagrant" to README.md

[life5ign]

Great plugin; I'm running Ubuntu 22.04 with ansible in a virtual environment, and from reading the README, it was not stated that I needed to pip install molecule-vagrant. When I tried to create a new role with the vagrant driver for molecule:

molecule init role --driver-name vagrant my_namespace.dummy_vagrant_driver_role

I got

Usage: molecule init role [OPTIONS] ROLE_NAME                                                                                                                                
Try 'molecule init role --help' for help.                                                                                                                                    
                                                                                                                                                                             
Error: Invalid value for '--driver-name' / '-d': 'vagrant' is not one of 'delegated', 'docker'. 

I finally stumbled upon (no link in the README) https://pypi.org/project/molecule-vagrant/ , and decided to give it a shot with pip. It worked!

googling "ansible plugins" didn't help, afaict. I didn't have time to run through the molecule code to see how it looks for drivers. So I don't know if this is an oddity on my system, or if this install step is missing from the README in this repo.