Remove "extern crate" directions from README

You no longer need to add `extern crate` directives for modern Rust editions.

Author: zmitchell

README.md

@@ -16,12 +16,6 @@ To use `glob`, add this to your `Cargo.toml`:
 glob = "0.3.1"
 ```
 
-And add this to your crate root:
-
-```rust
-extern crate glob;
-```
-
 ## Examples
 
 Print all jpg files in /media/ and all of its subdirectories.