update readme

Signed-off-by: mccm <mccleaner@126.com>

README.md

 # 常磐大逃杀（PHP Battle Royale）
 A PHP based game emulating the settings and gameplay of the original Battle Royale series, with a lot of original spin and an original story with tons of references to pop culture.
+# 安装
+需要php5或者7，composer（也可以不需要）。
+```bash
+git clone https://github.com/amarillonmc/phpdts.git
+cd phpdts
+composer install    #可选
+```
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