Update README.md

Author: zmitchell

README.md

@@ -16,6 +16,11 @@ To use `glob`, add this to your `Cargo.toml`:
 glob = "0.3.1"
 ```
 
+If you're using Rust 1.30 or earlier, or edition 2015, add this to your crate root:
+```rust
+extern crate glob;
+```
+
 ## Examples
 
 Print all jpg files in /media/ and all of its subdirectories.