Add small note for querying stocks on different exchanges

[flouthoc]

Hi @antirez

Following PR adds a small note on Readme to help users understand about

• How to query stocks on exchanges other than Nasdaq.

Thanks

[flouthoc]

Hi @antirez were you able to take a look at this.

[xmcqueen]

Check it out @antirez, it does cryptos too! This needs to be in the README too.

Try these:

$xmr-usd
$btc-usd