Added example for multiple callbacks

[pheraph]

Had to search the issue tracker to find out this is even possible.

[aminya]

@suda How can I create a sequential callback? When I use this example it seems it executes the 2nd command before the first one finishing.

[suda]

Unfortunately, this doesn't provide a way to control the execution order. You might create a single callback that then runs other callbacks in a sequential manner.

[aminya]

I wrote one example here if you want to add it to the readme:
https://github.com/aminya/juno-toolbar-plus/blob/60039275a1f07813cb0506f30b598c854bc9c4e2/lib/juno-toolbar-plus.coffee#L34-L40