readme - document using a custom assertion module

#25

readme.md

@@ -165,6 +165,20 @@ test.serial(function (t) {
 ```
 
 
+### Custom assertion module
+
+You can use any assertion module instead or in addition to the one that comes with AVA, but can't won't be able to use the `.plan()` method, [yet](https://github.com/sindresorhus/ava/issues/25).
+
+```js
+var assert = require('assert');
+
+test(function (t) {
+	assert(true);
+	t.end();
+});
+```
+
+
 ## API
 
 ### test([name], body)