Add example for $watch'ing $store to docs.

[james0r]

I see this is possible just like in Spruce, so it'd probably be of interest to others.

Did a demo to confirm it works -> https://codepen.io/James0r/pen/vYxPeqq?editors=1111

[zepfietje]

I think it's a good idea to add this to the docs indeed. Ended up here while trying to figure out how it works. Thanks @james0r!

<div x-init="$watch('$store.darkMode.on', value => console.log(value))"></div>

[ju5t]

This was doing my head in. Thanks for the solution @james0r and @zepfietje!