Doc: adds security warning for Command module in README

Author: CarterLi

README.md

@@ -104,6 +104,10 @@ Fastfetch uses JSONC (JSON with comments) for configuration. [See the Wiki for d
 
 Logos can also be heavily customized; see the [logo documentation](https://github.com/fastfetch-cli/fastfetch/wiki/Logo-options) for more information.
 
+### WARNING
+
+Fastfetch supports a `Command` module that can run arbitrary shell commands. If you copy-paste a config file from an untrusted source, it may contain malicious commands that can harm your system or compromise your privacy. Please always review the config file before using it.
+
 ## FAQ
 
 ### Q: Neofetch is good enough. Why do I need fastfetch?