Added readme

readme.md

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+#Example usage
+
+```extern crate quake3_loader;
+use std::fs::{File};
+use std::path::{Path};
+use std::io::{Read};
+
+fn main() {
+    let path = Path::new("pak0/maps/q3dm1.bsp");
+    let mut file = File::open(path).unwrap();
+    let mut bytes = Vec::new();
+    file.read_to_end(&mut bytes).unwrap();
+
+    let bsp = quake3_loader::read_bsp(bytes);
+           
+    println!("{:?}", bsp);
+}
+```