Point to docs.rs for documentation

(because Servo isn't always using the latest version of this crate)

Author: mbrubeck

README.md

@@ -1,6 +1,6 @@
 rust-smallvec
 =============
 
-[Documentation](http://doc.servo.org/smallvec/)
+[Documentation](http://docs.rs/smallvec/)
 
 "Small vector" optimization for Rust: store up to a small number of items on the stack