readme: added warning for cmdline leak

https://github.com/fphammerle/switchbot-mqtt/commit/5f7ed542c96a760e9f66039caaa36c5937b076c6#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5R112

README.md

@@ -26,6 +26,9 @@ $ ssh -p 2200 -T dump@localhost
 Git tags `docker/*` contain signed docker image digests:
 https://github.com/fphammerle/docker-mysqldump-sshd/tags
 
+⚠️  `MYSQLDUMP_ARGS=--password=...` leaks the password to other users on the same machine,
+if `/proc` is mounted with `hidepid=0` (default).
+
 ### Docker Compose 🐙
 
 1. `git clone https://github.com/fphammerle/docker-mysqldump-sshd`