I'm not sure if anyone is sure about what you're trying to do. It sounds like you're trying to obscure the url so that spammers, etc. won't link to your site.
Even if you change the url robots will still crawl those urls unless you prohibit them.
I don't believe this can be achieved with editing the plugin's readme.txt file. You can probably just delete that file as most plugins don't require them to function properly. In fact I recommend removing all readme.txt files everywhere as they are used to fingerprint (phishing) websites.
If you want to change the url it would probably be best done with RewriteRule via htaccess.