Add initial draft of subscriptions docs

This is an initial draft of some docs around subscriptions. Currently incomplete and pull requested to facilitate discussion.

###Todo

- [] Split out connection events into their own doc
- [] Comparison of types of subscription and their guarantees
- [] Canonical patterns for subscription set up
- [] Document regular subscriptions API
- [] Document catch-up subscriptions API
- [] Document persistent subscriptions API (this may be a separate job as it's a competing consumers feature)