Add README

Add benchmark result

Author: Jason Young

README.md

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+# php-rapidjson
+PHP Extension of rapidjson
+
+#Result of benchmark(gcc -O3) [Script](https://github.com/RustJason/php-rapidjson/blob/master/rapidjson.php):
+```bash
+json.len = 253438
+Rapidjson.time: 1.4133679866791
+Rapidjson.mem: 96
+json_decode.time: 9.0703821182251
+json_decode.mem: 1530672
+```
+
+