add note about synchronicity

docs/render.md

@@ -1,7 +1,8 @@
 # render(element, vnodes)
 
 - [API](#api)
-- [How it works](#how it works)
+- [How it works](#how-it-works)
+- [Why Virtual DOM](#why-virtual-dom)
 - [Differences from other API methods](#differences-from-other-api-methods)
 - [Standalone usage](#standalone-usage)
 
@@ -23,7 +24,13 @@ Argument    | Type                 | Required | Description
 
 ### How it works
 
-The `m.render(element, vnodes)` method takes a virtual DOM tree (typically generated via the [`m()` hyperscript function](hyperscript.md), generates a DOM tree and mounts it on `element`. If `element` already has a DOM tree mounted via a previous `m.render()` call, `vnodes` is diffed against the previous `vnodes` tree and the existing DOM tree is modified only where needed to reflect the changes. Unchanged DOM nodes are not touched at all.
+The `m.render(element, vnodes)` method takes a virtual DOM tree (typically generated via the [`m()` hyperscript function](hyperscript.md), generates a DOM tree and mounts it on `element`. If `element` already has a DOM tree mounted via a previous `m.render()` call, `vnodes` is diffed against the previous `vnodes` tree and the existing DOM tree is modified only where needed to reflect the changes. Unchanged DOM nodes are not touched at all. 
+
+`m.render` is synchronous.
+
+---
+
+### Why Virtual DOM
 
 It may seem wasteful to generate a vnode tree on every redraw, but as it turns out, creating and comparing Javascript data structures is surprisingly cheap compared to reading and modifying the DOM.