readme: clarify mojangProxy option

inspired by #88

README.en.md

@@ -53,6 +53,9 @@ Configure Minecraft server with the following JVM parameter:
     Only SOCKS protocol is supported.
     URL format: socks://<host>:<port>
 
+    This proxy setting only affects Mojang namespace feature, and the proxy is used only when accessing Mojang's servers.
+    To enable proxy for your customized authentication server, see https://docs.oracle.com/javase/8/docs/technotes/guides/net/proxies.html .
+
 -Dauthlibinjector.legacySkinPolyfill={default|enabled|disabled}
     Whether to polyfill legacy skin API, namely 'GET /skins/MinecraftSkins/{username}.png'.
     It's enabled by default if the authentication server does NOT send feature.legacy_skin_api option.

README.md

@@ -52,6 +52,9 @@ gradle
     设置访问 Mojang 验证服务时使用的代理, 目前仅支持 SOCKS 协议.
     URL 格式: socks://<host>:<port>
 
+    这一代理仅作用于 Mojang 命名空间 功能, 其仅用于访问 Mojang 服务器.
+    若要在访问自定义验证服务器时使用代理, 请参考 https://docs.oracle.com/javase/8/docs/technotes/guides/net/proxies.html .
+
 -Dauthlibinjector.legacySkinPolyfill={default|enabled|disabled}
     是否启用旧式皮肤 API polyfill, 即 'GET /skins/MinecraftSkins/{username}.png'.
     若验证服务器未设置 feature.legacy_skin_api 选项, 则该功能默认启用.