If you are under MacOS, as said in the README.txt (but you need to already be able to unarchive it to access it), you have to get an unarchiver to… well, unarchive it and the README.txt give a link to The Unarchiver @ Mac App Store.
Otherwise you can type in .rar in your favorite search engine and you will see some links to WinRAR, WinZip, 7-Zip and maybe some others.