put security disclaimer in README

Author: FauconFan

README.md

@@ -16,6 +16,13 @@ You also have to activate the preprocessor, put this in your `book.toml` file:
 [preprocessor.cmdrun]
 ```
 
+> :warning: This preprocessor presents a security risk, as arbitrary commands can be run. Be careful with the commands you run.
+> To list all the commands that will be run within an mdbook, you can run the following command:
+> ```sh
+> grep -r '<!-- cmdrun' . | sed 's/.*<!-- cmdrun \(.*\) -->.*/\1/'
+> ``````
+
+
 ## How to
 
 Let's say we have these two files:

src/lib.rs

@@ -11,6 +11,13 @@
 //! [preprocessor.cmdrun]
 //! ```
 //!
+//! > :warning: This preprocessor presents a security risk, as arbitrary commands can be run. Be careful with the commands you run.
+//! > To list all the commands that will be run within an mdbook, you can run the following command:
+//! > ```sh
+//! > grep -r '<!-- cmdrun' . | sed 's/.*<!-- cmdrun \(.*\) -->.*/\1/'
+//! > ``````
+//!
+//!
 //! # How to
 //!
 //! Let's say we have these two files: