Explain how to set up CORS header in the app

[fsdiogo]

If a user as a running daemon but with no CORS configured, this will appear for his confusion:

We must show how to configure CORS to make everything crystal clear.

One way of doing it: https://github.com/ipfs-shipyard/ipfs-webui/pull/839

References:

• https://github.com/ipfs-shipyard/ipfs-webui/issues/817