[Docs] Update TimerMixin docs.

Adds in install instructions for `react-timer-mixin` to the `TimerMixin` docs.

docs/Timers.md

@@ -57,6 +57,8 @@ InteractionManager.clearInteractionHandle(handle);
 
 We found out that the primary cause of fatals in apps created with React Native was due to timers firing after a component was unmounted. To solve this recurring issue, we introduced `TimerMixin`. If you include `TimerMixin`, then you can replace your calls to `setTimeout(fn, 500)` with `this.setTimeout(fn, 500)` (just prepend `this.`) and everything will be properly cleaned up for you when the component unmounts.
 
+This library does not ship with React Native - in order to use it on your project, you will need to install it with `npm i react-timer-mixin --save` from your project directory.
+
 ```javascript
 var TimerMixin = require('react-timer-mixin');