Auto merge of #9000 - JohnTitor:owner-add-docs, r=ehuss

Document `could not find the github team` error on `cargo owner --add`

When running `cargo owner --add`, the `could not find the github team` error often occurs due to lack of the permissions.
Example cases: https://github.com/rust-lang/cargo/issues/5297#issuecomment-653709183, https://github.com/rust-lang/crates.io/issues/2928

I think it's worth documenting that error and the possible solution.