README.md: Add overview and example

README.md

@@ -1 +1,39 @@
 # servant-activeresource
+
+[ActiveResource](https://github.com/rails/activeresource) is a Rails
+library for representing resources from a RESTful API as Ruby objects,
+with a similar interface to the Rails ActiveRecord ORM.
+
+This library provides types and TH helpers for describing such APIs,
+and for implementing Servant-style servers to provide them.
+
+```haskell
+{-# LANGUAGE TemplateHaskell #-}
+
+import qualified Servant.ActiveResource as AR
+
+newtype MyResourceId = MyResourceId Int
+-- Type for new values or updates to existing values. Usually
+-- missing an @id@ field.
+data MyResource = MyResource {...}
+-- Like MyResource, but returned from the database.
+data MyStoredResource = MyStoredResource {...}
+
+-- The exact monad used will depend on your program. Here, we just assume
+-- Handler from package servant-server.
+instance AR.Resource MyResourceId Handler where
+  type ResourceData MyResourceId = MyResource
+  type StoredResourceData MyResourceId = MyStoredResource
+
+  -- These form the implementation of your API.
+  listResources = ...
+  createResource = ...
+  readResource = ...
+  upsertResource = ...
+  deleteResource = ...
+
+-- Record of routes, which can be spliced into a top-level handler
+-- via Servant.API.NamedRoutes.
+routes :: AR.ResourceRoutes MyResourceId (AsServerT Handler)
+routes = $(AR.makeResourceServerT [t|MyResourceId|])
+```