Mention how to add new assets in README

[xaur]

Someone asked in #dex chat where are the docs for integrating new assets. The answer was

https://github.com/decred/dcrdex/blob/master/spec/fundamentals.mediawiki/#adding-new-assets
https://github.com/decred/dcrdex/tree/master/server/asset

I think (hope) this will become a popular question and putting it in the README would help.