In the module you are uploading to the appstore add a folder named Readme. In that folder place a snippet called readme and in there you place the text for the people to read.
The readme file is indeed just a snippet in which you place a text element. If it is any more the a couple of lines, editting it in a text-element is very cumbersome. You will be much happier to copy-paste from-to your favorite text-editor.