Document sample usage vanilla, turbolinks, and turbograft

Adding some usage notes in different operating scenarios

cc @pushrax do these look fairly accurate?  I trimmed down the Turbograft usage notes (since we have some re-application of old context stuff, that isn't necessarily directly applicable)

Here's a link to the nicely formatted README that this PR introduces: https://github.com/Shopify/twine/blob/b811802fd2401d6bde07c22e19f93fcf9449fb7f/README.md