Two ideas:
Use master in the url (this seems to work):
https://bitbucket.org/wordless/thofu-interpreter/raw/master/ThoFu%20Interpreter/ReadMe.txt
Another idea is to create a wiki page for your project, then use the wiki’s functionality to link to the latest version of a file with this syntax:
<<file path/to/file [revision] [linenumber]>>
Just omit the revision and line number parameters, they are optional.
Then, refer to the wiki page everywhere else you’d like link to the read me file.