Update README with missing setup steps

[CloudNiner]

• Add app_assets step
• Add note about sql api/windshaft master branches to config section
• Cleanup formatting

[hectcastro]

👍

I went through all of these step yesterday, minus Copy CartoDB app configuration (optional) and things were in working order (also, after https://github.com/azavea/vagrant-cartodb/pull/4).

[CloudNiner]

Yeah, internally, the provisioner copies the app_config.yml for you, but only if it doesn't already exist