Docs: add an additional example for triggering only when input length >= 3

[mristin]

Hi,
It is a very common scenario to trigger a search only when the input length is above a certain limit (say, 3 characters), and otherwise show a message that the input is too short.

I think this would be a tremendous help for the newbies, in addition to the example where you trigger with a delay.

Please let me know if you want me to create a pull request.

[1cg]

Yes please. A good example might be a keyup:

https://codepen.io/1cg/pen/dydpQGj?editors=1001