Add usage description for proxy_auth parameter

Author: pikadoramon

README.md

@@ -66,6 +66,8 @@ Usage of go-mitmproxy:
     	map local config filename
   -map_remote string
     	map remote config filename
+  -proxyauth string
+        enable proxy authentication. Format: "username:pass", "user1:pass1|user2:pass2","any" to accept any user/pass combination
   -ssl_insecure
     	not verify upstream server SSL/TLS certificates.
   -upstream string

README_CN.md

@@ -66,6 +66,8 @@ Usage of go-mitmproxy:
     	map local json配置文件地址
   -map_remote string
     	map remote json配置文件地址
+  -proxyauth string
+        启用代理认证。格式："user:pass"、"user1:pass1|user2:pass2"，或使用 "any" 允许所有用户
   -ssl_insecure
     	不验证上游服务器的 SSL/TLS 证书
   -upstream string