README updates

[MaggieCabrera]

Let's do some upkeep and make the Readme more useful for new contributors. In the future part of it might move to the Themes team's handbook but in the meantime, let's add a section for Contributor days, both for organizers and attendees, so they can understand how to contribute and how the repo works.

[MaggieCabrera]

Let's update the playground instructions with a visual guide for people that don't have a development environment