Add a reason to the libc & rand instability.

Many many many people ask in #rust about this libraries, having an
explanatory reason will probably help a lot.

Author: huonw

src/liblibc/lib.rs

@@ -12,7 +12,8 @@
 #![cfg_attr(stage0, feature(custom_attribute))]
 #![crate_name = "libc"]
 #![crate_type = "rlib"]
-#![cfg_attr(not(feature = "cargo-build"), unstable(feature = "libc"))]
+#![cfg_attr(not(feature = "cargo-build"), unstable(feature = "libc",
+                                                   reason = "use `libc` from crates.io"))]
 #![cfg_attr(not(feature = "cargo-build"), feature(staged_api, core, no_std))]
 #![cfg_attr(not(feature = "cargo-build"), staged_api)]
 #![cfg_attr(not(feature = "cargo-build"), no_std)]

src/librand/lib.rs

@@ -26,7 +26,8 @@
        html_playground_url = "http://play.rust-lang.org/")]
 #![no_std]
 #![staged_api]
-#![unstable(feature = "rand")]
+#![unstable(feature = "rand",
+            reason = "use `rand` from crates.io")]
 #![feature(core)]
 #![feature(no_std)]
 #![feature(staged_api)]