It would be nice, if we dont consider the fact that nobody reads README at all.
"A perfect implementation of the wrong specification is worthless."
The point of BDD is exactly this: bring specification to the scene, give it a first class citizen status, it becomes a live entity in the system, more influential than before. I don't believe Readme or any other document can be as influential as a Cucumber feature, for instance.
When I'm researching new tools/libraries, the first thing I read is the README. If it's worthless, then it's a fair bet the tool/library is, as well, and I won't be using it by choice. After that, I look at the official documentation. If all it has is auto-generated API documentation, that's another strike against it. Then I look at the API documentation. The tests are somewhere way in the back after all of this.
The point is that writing one is valuable -- it tests your ability to articulate an understanding of your own work from a high level, which is not always obvious when our focus is frequently at the code level.