Add some preliminary docs

CHANGELOG.md

@@ -3,6 +3,10 @@
 
 ## Upcoming
 
+- Reagent no longer bundles React. Instead it uses cljsjs/react as a dependency. This means that you should no longer specify React in `:preamble` in your project.clj.
+
+- `cursor` is re-written, to be more efficient and flexible.
+
 - `render` now forces a deep update of all components, to make it more convenient to use with e.g. figwheel.
 
 - Renamed `as-component` to `as-element`, to match React's new terminology better (old name still works, though, for backward compatiblity).

src/reagent/core.cljs

@@ -212,10 +212,22 @@ the specified path within the wrapped Reagent atom. e.g.,
     ... (reset! c 42) ;; equivalent to (swap! ra assoc-in [:nested :content] 42)
     ... (swap! c inc) ;; equivalence to (swap! ra update-in [:nested :content] inc)
     )
+
+The first parameter can also be a function, that should look something
+like this:
+
+(defn digit-filter
+  ([k] (get-in @state k))
+  ([k v] (swap! state assoc-in k v)))
+
+The function will be called with one argument – the path passed to
+cursor – when the cursor is deref'ed, and two arguments (path and new
+value) when the cursor is modified.
 "
   ([src path]
    (ratom/cursor src path)))
 
+
 ;; Utilities
 
 (defn next-tick