Add initial alpha instructions document.

docs/2.0-alpha-instructions.md

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+# Alpha Instructions
+
+## What to expect
+
+The API is not yet stable, and there will be breaking changes between now and the public 2.0 release.
+
+There may be some remaining bugs, but overall the current alpha appears to be relatively stable.
+
+There isn't a lot of documentation beyond the examples folder and this document right now. Part of the alpha process will be
+getting the documentation ready, and changing APIs wherever things appear to be confusing or have API improvement opportunities.
+
+Feedback and documentation improvements are very welcome.
+
+### Special files
+
+- `Document.elm` - defines the types for your applications view. Can expose any type called `Document msg` (must have exactly one type variable)
+- `Shared.elm` must expose
+  - `template : SharedTemplate Msg Model StaticData msg`
+  - `Msg`
+  - `Model`
+  - `SharedMsg` (todo - this needs to be documented better. Consider whether there could be an easier way to wire this in for users, too)
+- `Site.elm` - must expose
+  - `config : SiteConfig StaticData`
+- `static/index.js` - same as previous `beta-index.js`
+- `static/style.css` - same as previous `beta-style.css`
+
+## Available generated code
+
+- `Route.elm` - lets you refer to a Route, and turn it into a path
+  - Every variant in `Route` has a record of `RouteParams`, which match the `type alias RouteParams = { ... }` in the corresponding Page Template
+  - Each Route has RouteParams - some of them empty, some of them non-empty - each param is a `String`, and the field key for each param corresponds to a dynamic segment (ending in `_`) in the Page Template's module name. For example, `Template.Slide.Number_` has a single route param, so it has `type alias RouteParams = { number : String }`
+  - You can link or navigate to a Route programatically with `firstSlide = Route.Slide__Number_ { number = "1" }`
+  - You can turn it to a String with `"/" + (Route.routeToPath firstSlide |> String.join "/")`
+
+### Commands
+
+`node ../../generator/src/dev-server.js`
+`../../generator/src/codegen-template-module.js Slide.Number_`