Update documentation to point to Rust issue

Author: josephlr

README.md

@@ -35,8 +35,10 @@ libc = "0.2"
 * `extra_traits`: all `struct`s implemented in `libc` are `Copy` and `Clone`.
   This feature derives `Debug`, `Eq`, `Hash`, and `PartialEq`.
 
-* `unstable`: enable currently unstable bindings. Right now, this just allows
-  bindings to `#[thread_local]` statics on certain platforms. Requires nightly.
+* `unstable`: This feature enables `libc` bindings that are only possible with
+  unstable Rust features. Right now, this just for
+  [`extern` `#[thread_local]` statics](https://github.com/rust-lang/rust/issues/29594)
+  on certain platforms. Requires nightly.
 
 * **deprecated**: `use_std` is deprecated, and is equivalent to `std`.