docs: Clarify usage of readonly for dynamic variables/functions

README.md

@@ -18,7 +18,8 @@ These are general guidelines for writing official bash modules and their documen
 
 - Echo what you're doing on each step and on errors to help debugging.
 - Implement error-checks for scenarios where the image-maintainer might misconfigure the module.
-- Use `snake_case` for functions and variables changed by the code.
+- Use `snake_case` for functions and variables changed by the code.  
+  You can utilize `readonly (-f) snake_case` to mark variable or function as read-only when it won't be changed anymore.
 - Use `readonly SCREAMING_SNAKE_CASE` for variables that are set once and stay unchanged.
 - Use `"${variable_name}"` when you want to expose information from the variable & to ensure that variables are properly parsed as strings.
 - If you want to insert another regular string as a suffix or prefix to the `"${variable_name}"`, you should do that in this format: `"prefix-${variable_name}-suffix"`