Add example for serializing Entity structure

examples/serialize.rs

@@ -0,0 +1,10 @@
+use std::{env, fs};
+use wikidata::*;
+
+fn main() {
+    let mut args = env::args();
+    let _ = args.next();
+    let contents = fs::read_to_string(args.next().unwrap()).unwrap();
+    let ent = Entity::from_json(serde_json::from_str(&contents).unwrap()).unwrap();
+    println!("{}", serde_json::to_string_pretty(&ent).unwrap());
+}