[add] Docs for stores

docs/store.md

@@ -0,0 +1,78 @@
+# Stores
+
+The responsibility of a Store in Microcosm is to transform state from
+one form to the next. They do not provide any storage of their own
+(although technically this isn't enforced by Microcosm).
+
+Stores boil down to:
+
+```javascript
+var store = {
+  getInitialState() {
+    // return a starting value
+  },
+  serialize(state) {
+    // transform data when app.toJSON() is called
+  },
+  deserialize(state) {
+    // parse data when app.seed is called
+  },
+  toString() {
+     // return a unique identifier
+  }
+}
+```
+
+Of these methods, only the `toString` method is required. If this
+method is not implemented Microcosm will quickly throw an error:
+
+```
+Stores must implement a toString() method
+```
+
+## Installing Stores
+
+Microcosms must add stores:
+
+```javascript
+let app = new Microcosm()
+
+let MyStore = {
+  toString() {
+    return 'mystore'
+  }
+}
+
+app.addStore(MyStore)
+```
+
+This will mix the given store on top of a set of defaults (see
+`src/Store.js`) and run `getInitialState()` if it is
+provided. Additionally, the Micocosm instance will now be configured
+to use `MyStore` to manage state under the `mystore` key (because of
+the `toString` method).
+
+This state can be accessed like:
+
+```javascript
+app.get(MyStore)
+```
+
+## Listening to Actions
+
+Stores listen to actions by implement them as methods:
+
+```javascript
+let MyStore = {
+  [Action.add](state, record) {
+    return state.concat(record)
+  },
+  toString() {
+    return 'my-store'
+  }
+}
+```
+
+The first argument of this method will always be the application state
+for the given key provided by `toString()`. It is the responsibility
+of the store to return the next state as a result of actions.