Add a README explaining how people can use Chronik.

Currently instructs to use the NNG interface, which is the most stable.

In the future, this will describe the steps necessary to set up Chronik within Bitcoin ABC.

Also add a cute logo by Alita Yin.

Look at https://github.com/raipay/bitcoin-abc/tree/chronik-readme/chronik and see if it renders correctly (or any other repository)