[suggestion] add usage instructions to README

[ES-Alexander]

1. At the moment it's not clear where users should create their workspaces
   • ideally this would be in the /extensions/<extension-name>/ directory (perhaps in a projects folder?), although
     • that will require an update to the Dockerfile binds, and
     • is probably best to wait until Folder extensions bluerobotics/BlueOS#2010 is merged
2. It's not obvious which (if any) libraries are included, and how to install new ones
   • I can potentially make a setup walkthrough video or something, but basic written docs would also be good as an overview

I would have submitted a PR for this, but I don't know the answers at the moment.