Add test method documentation to README

README.md

@@ -36,7 +36,7 @@ inView('.someSelector')
 
 ## API
 
-in-view maintains a separate handler registry for each set of elements captured with `inView(<selector>)`. Each registry exposes the same four methods. in-view also exposes three top-level methods. (`is`, `offset`, `threshold`).
+in-view maintains a separate handler registry for each set of elements captured with `inView(<selector>)`. Each registry exposes the same four methods. in-view also exposes four top-level methods. (`is`, `offset`, `threshold`, `test`).
 
 ### inView(\<selector>).on(\<event>, \<handler>)
 > Register a handler to the elements selected by `selector` for `event`. The only events in-view emits are `'enter'` and `'exit'`.
@@ -85,7 +85,15 @@ in-view maintains a separate handler registry for each set of elements captured
 > inView.threshold(0);
 > inView.threshold(0.5);
 > inView.threshold(1);
-> ``` 
+> ```
+
+### inView.test(\<test>)
+> Override in-view's default visibility criteria with a custom function. This function will receive the element and the options object as its only two arguments. Return `true` when an element should be considered visible and `false` otherwise.
+> ```js
+> inView.test((el, options) => {
+>     // ...
+> });
+> ```
 
 ### inView(\<selector>).check()
 > Manually check the status of the elements selected by `selector`. By default, all registries are checked on `window`'s `scroll`, `resize`, and `load` events.