Actions doc

***COMMENTS REQUESTED*** (especially from frontend hackers, see item 3)

 -- disclaimer: this PR is just for the purpose of getting some discussion flowing -- 


I think there are four main consumers with different needs that this doc and/or other docs should address.

1. Juju Users
 - This is probably mostly addressed by @jcw4 in https://github.com/juju/docs/pull/117 `WIP`
 - I think we should probably add a UX definition so it's in the juju/docs folder. `TODO`
2. Charm Creators
 - Charm Creators need to know how to define an Action on the Charm. They need to know:
   - How to define a Charm Action script (i.e. it's a Hook) `TODO`
    - How the Charm Action will interact with the Hook environment (e.g. ActionGet) `TODO`
    - The requirement to create actions.yaml to specify the Charm's Actions' parameters. `DONE`
3. Frontend Juju Hackers
 - They need a clear spec for the client API function by function. `TODO`
 - I'd like to get input from Rick and Jeremy what API endpoints the frontend team needs. `TODO`
   - Actions-Schema getter? (serialized JSON-schema for GUI construction?)
    - What kind of data comes back from juju queue? What about juju status?
    - They need a clear UX spec for various possible interactions.
    - What API endpoints are synchronous or asynchronous?
 - Other GUI questions should be discussed in mailing list with Frontend Hackers. `TODO`
4. Backend Juju Hackers
 - A sketch from client API to State to Hook and back. `TODO`
 - A sketch for Charm Actions. Technical clarity between: `WIP`
   - Actions schema (i.e. Charm Actions, and gojsonschema's role) `DONE`
    - Lifecycle of Actions in State, and how Watchers / Uniters relate `WIP`
    - Action hooks and their operation `TODO`
    - Action responses and the Action log `TODO`