pub fn readme(&self) -> Option<&'g Utf8Path> {
        self.inner.readme.as_ref().map(|path| path.as_ref())
    }
Returns a path to the README for this package, if specified.

This is the same as the `readme` field of `Cargo.toml`. The path returned is relative to the
directory the `Cargo.toml` is in (i.e. relative to the parent of `self.manifest_path()`).
guppy/src/graph/graph_impl.rs