docs: add docs.rs flag for feature annotations (#13)

### [`doc_auto_cfg`: Automatically generate
`#[doc(cfg)]`](https://doc.rust-lang.org/rustdoc/unstable-features.html#doc_auto_cfg-automatically-generate-doccfg)

-   Tracking issue:
[#43781](rust-lang/rust#43781)

`doc_auto_cfg` is an extension to the `#[doc(cfg)]` feature. With it,
you don't need to add `#[doc(cfg(...)]` anymore unless you want to
override the default behaviour.

src/lib.rs

@@ -10,6 +10,7 @@
     clippy::cargo_common_metadata,
     clippy::multiple_crate_versions
 )]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![doc = include_str!("../README.md")]
 mod stdx;