Delete README.txt and link to fiati-usage.md instead

[Stephen Buley]

I found it confusing upon finding the repo that there were two README files in the root directory, one README.md which was displayed in the normal github way, and another README.txt that was linked to in the original file. I think it makes more sense to link to the fiati-usage.md file and delete the README.txt which is essentially a copy. Another route would be to copy/paste the usage docs into the README.md, but that defeats the purpose of having a separate usage doc altogether.