Currently ReadMe! supports the .epub-format and PDF, so the short answer for TXT or other formats would be "You can't".
But if you have a mindset like "Nothing is impossible!" then you can try to convert it to .epub. This works pretty well most of the times, but often the converted file will lack chapter information and such so it's not a 100% satisfaction guaranteed when converting files.
There are some free services that can transform many formats to .epub, just by uploading them to a website and then they will do all the work and supply you with a download link.
One of these services is http://www.2epub.com that can convert a range of files to Epub. Please note that we have no affiliation with 2epub.com so if there is a problem with their service, we can't help you other than suggest that you change service to another one. We found this one simply by going to google and searching for "pdf to epub".
Also, if you really want to browse the converted book by chapters or if there is something else that you want to change in the book you can get the free software Calibre at http://calibre-ebook.com .
This is an EPUB editor that's widely used and appreciated. It gives you the possibility to quickly edit your book files, add chapters, change book-covers and more. But again, we have no affiliation with Calibre either, we just think it's a great piece of software that we want you to know about. If you need help with Calibre, please direct your questions to the Calibre help sections as we can't answer any questions about it.