Copy documentation of semver exempt to rustdoc

Author: dtolnay

src/lib.rs

@@ -20,6 +20,25 @@
 //! This'll allow immediate usage of the beneficial upstream API, particularly
 //! around preserving span information.
 //!
+//! # Unstable Features
+//!
+//! `proc-macro2` supports exporting some methods from `proc_macro` which are
+//! currently highly unstable, and may not be stabilized in the first pass of
+//! `proc_macro` stabilizations. These features are not exported by default.
+//! Minor versions of `proc-macro2` may make breaking changes to them at any
+//! time.
+//!
+//! To enable these features, the `procmacro2_semver_exempt` config flag must be
+//! passed to rustc.
+//!
+//! ```sh
+//! RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
+//! ```
+//!
+//! Note that this must not only be done for your crate, but for any crate that
+//! depends on your crate. This infectious nature is intentional, as it serves
+//! as a reminder that you are outside of the normal semver guarantees.
+//!
 //! [`proc_macro`]: https://doc.rust-lang.org/proc_macro/
 //! [ts]: https://doc.rust-lang.org/proc_macro/struct.TokenStream.html