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/add-usage-notes/README.md
At first I wondered if the "getting started" snippet would be simpler if it was just `{}`, but I see after reading more of it that the name `context` gets re-used a lot and that's what this is. I think this is ok, you can ignore this comment.