Document cargo doc --document-private-items in README

[tomhoule]

[h-michael]

@tomhoule
Could you elaborate a little more on your intention of this issue?

[tomhoule]

Sorry I didn't reply earlier - there is a --document-private-items flag you can pass to cargo doc so the code generated by the derive is documented, even if it's private.

[tomhoule]

It's now in the README so I'll close this issue.