link to reactive-banana-example from README

also fix a typo.
clarify: it's really a toy program.
A screenshot of the toy app
center the image
Revert "center the image"

The <center> tag doesn't work on GitHub :(

This reverts commit b038d0d731675cb8bfd78fe6cc6dfadbba9a4e0e.
minimum reactive-banana version

obtained via cabal-rangefinder
callback example
link to callback-example from README