Mention that -D warnings will deny ALL warnings, not just clippy warnings

Author: sunjay

README.md

@@ -125,6 +125,11 @@ before_script:
    # etc
 ```
 
+Note that adding `-D warnings` will cause your build to fail if **any** warnings are found in your code.
+That includes warnings found by rustc (e.g. `dead_code`, etc.). If you want to avoid this and only cause
+an error for clippy warnings, use `#![deny(clippy::all)]` in your code or `-D clippy::all` on the command
+line. (You can swap `clippy::all` with the specific lint category you are targetting.)
+
 ## Configuration
 
 Some lints can be configured in a TOML file named `clippy.toml` or `.clippy.toml`. It contains a basic `variable = value` mapping eg.