You don’t have to put out a new version of your plugin to update the readme.
I do this all the time because I don’t develop on SVN. I get my whole plugin over onto the SVN repo for the WordPress.org repository. Make a big update. Then I realise I completely forgot to add some detail to the readme.
The first couple times I did this I just bumped the version number. But you don’t have to do this. You can just update the readme.txt in the current stable tagged version’s directory. So if you’re on 1.2, you’d edit
/plugin-slug/tags/1.2/readme.txt and then check in your changes.
It can take a minute or two to update, but it will update. If you don’t have a
/tags/ directory in the local copy of your SVN repo, you may need to run
svn up to pull the tags down from the repo.