message: Set default for readme in Manifest if it's not specified.
  If the readme field is not specified, assume a default of
  "README.md" (temporarily). Further, if the readme field is set to
  "false", then this defaulting behavior is suppressed.
diff: @@ -1198,11 +1198,20 @@ impl TomlManifest {
  project.links.as_deref(),
  project.namespaced_features.unwrap_or(false),
  )?;
  +
  + let readme = match &project.readme {
  + None => Some(String::from("README.md")),
  + Some(value) => match value.as_str() {
  + "false" => None,
  + _ => Some(value.clone())
  + }
  + };
  +
  let metadata = ManifestMetadata {
  description: project.description.clone(),
  homepage: project.homepage.clone(),
  documentation: project.documentation.clone(),
  - readme: project.readme.clone(),
  + readme,
  authors: project.authors.clone().unwrap_or_default(),
  license: project.license.clone(),
  license_file: project.license_file.clone(),
  