Update README and CLI commands

[douglascayers]

• Removed force:package2 commands from README
• Updated metadata api to at least v44.0
• Added link back to the Trailhead project

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @douglascayers to sign the Salesforce.com Contributor License Agreement.

[ekapner]

Hey @douglascayers - just curious why you aren't also "enabling" chatter and Lightning, per the original scratch def.

[douglascayers]

Hi @ekapner, I left those out because they weren't necessary, at least not in my tests. New orgs come with Chatter and Lightning already enabled. It doesn't hurt to include them though to be overly explicit.