Add Demo / Example

[romellem]

I played around with this a tiny bit and made a small demo

• https://codesandbox.io/s/flow-field-example-914pz

Feel free to copy the code and include in the repo, or if the demo doesn't showcase what the library is meant to do, feel free to create a new demo.

[romellogoodman]

Yo! Thanks so much for making this. I just added a codesandbox example to the readme that has a little bit more interactivity. Check it out!