Add credit for API usage

We're using the https://min-api.cryptocompare.com/ API, so we need to give credit:

> Our API is under a Creative Commons - Attribution Non-Commercial license. This means that it’s free as long as you attribute it to us and you are not making money from the data. If you are selling a product that uses our data we charge a fee. The idea is that if you start making a lot of money using our data, we would like something in return. If you are just providing a free service, you can use our data freely. **Please make sure you credit us with a link if you use our data on your website or app.**

We should either give credit on the HyperDEX website or in the about dialog in the app.