message: Assert preconditions for README copying in `readme::copy_from_crate`
  The implementation relies on the crate directory existing, and the `pkg`
  directory in the crate directory existing as well.
diff: @@ -10,6 +10,17 @@ use PBAR;
  /// Copy the crate's README into the `pkg` directory.
  pub fn copy_from_crate(path: &Path, step: &Step) -> Result<(), Error> {
  + assert!(
  + fs::metadata(path).ok().map_or(false, |m| m.is_dir()),
  + "crate directory should exist"
  + );
  + assert!(
  + fs::metadata(path.join("pkg"))
  + .ok()
  + .map_or(false, |m| m.is_dir()),
  + "crate's pkg directory should exist"
  + );
  +
  let msg = format!("{}Copying over your README...", emoji::DANCERS);
  PBAR.step(step, &msg);
  let crate_readme_path = path.join("README.md");
  