info: Update license information in custom-vec

Author: ezrosent

info/custom-vec.md

@@ -1,3 +1,9 @@
+<!-- Copyright 2017 the authors. See the 'Copyright and license' section of the
+README.md file at the top-level directory of this repository.
+
+Licensed under the Apache License, Version 2.0 (the LICENSE-APACHE file) or
+the MIT license (the LICENSE-MIT file) at your option. This file may not be
+copied, modified, or distributed except according to those terms. -->
 # Fun with the Rust `Alloc` trait
 
 This post gives a brief tour of Rust's new allocation interface, and some