Updated README.MD with a blurb about how Hotkeys must have a descript…

…ion to show up in the cheat sheet.

In reference to issue #63

README.MD

@@ -65,6 +65,18 @@ Based off of the [angular-hotkeys library](https://github.com/chieffancypants/an
 To enable the cheat sheet, simply add `<hotkeys-cheatsheet></hotkeys-cheatsheet>` to your top level component template.
 The `HotkeysService` will automatically register the `?` key combo to toggle the cheat sheet.
 
+**NB!** Only hotkeys that have a description will apear on the cheat sheet. The Hotkey constructor takes a description as
+an optional fourth parameter as a string or optionally as a function for dynamic descriptions.
+
+```typescript
+    this._hotkeysService.add(new Hotkey('meta+shift+g', (event: KeyboardEvent): boolean => {
+        console.log('Secret message');
+        return false;
+    }, undefined, 'Send a secret message to the console.'));
+```
+
+The third parameter, given as `undefined`, can be used to allow the Hotkey to fire in INPUT, SELECT or TEXTAREA tags.
+
 ### Cheat Sheet Customization
 
 1. You can now pass in custom options in `HotkeysModule.forRoot(options: IHotkeyOptions)`.