Clarify documentation on features

[GideonBear]

In 0.42 I used no_default_features = true, features = ["rustls"] which doesn't work now that reqwest or ureq is required. Let's clarify this in the README.

It fails with:

error[E0425]: cannot find function `get` in module `http_client`
   --> src/backends/gitlab.rs:555:33
    |
555 |         let resp = http_client::get(&api_url, self.api_headers(&self.auth_token)?)?;
    |                                 ^^^ not found in `http_client`