Document password limitation in README

Added warning about passwords containing spaces, double quotes,
or $ characters potentially failing the build.

README.md

@@ -285,6 +285,9 @@ Use the provided build script:
 - SMTP username and password
 - SSL/TLS setting (default: yes)
 
+**⚠️ Password Limitation:**
+The build script uses simple shell variable substitution for credentials. Passwords containing spaces, double quotes (`"`), or dollar signs (`$`) may cause the build to fail. If your password contains these characters, consider using a simpler password or manually editing the build command.
+
 ---
 
 ## Testing