docs: show the required feature on docs.rs (#209)

see https://docs.rs/about/builds#detecting-docsrs
see https://doc.rust-lang.org/rustdoc/unstable-features.html#doc_auto_cfg-automatically-generate-doccfg

While other projects seem to `--cfg docsrs` in the Cargo.toml this is likely not required (anymore?) as its documented as the way to detect docs.rs

src/lib.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+
 #[doc(hidden)]
 #[cfg(feature = "async-http-client")]
 pub use reqwest;