message: Update documentation for the readme field
diff: @@ -165,6 +165,11 @@ will interpret it as Markdown and render it on the crate's page.
  readme = "README.md"
  ```
  +If no value is specified for this field, and a file named `README.md`,
  +`README.txt` or `README` exists in the package root, then the name of that
  +file will be used. You can suppress this behavior by setting this field to
  +`"false"`.
  +
  #### The `homepage` field
  The `homepage` field should be a URL to a site that is the home page for your
  