readme: show how to make the travis job fail when using clippy

README.md

@@ -82,10 +82,15 @@ You can add Clippy to Travis CI in the same way you use it locally:
 
 ```yml
 - rust: stable
+- rust: beta
   before_script:
     - rustup component add clippy-preview
   script:
     - cargo clippy
+# if you want the build job to fail when encountering warnings, use
+    - cargo clippy -- -D warnings
+# in order to also check tests and none-default crate features, use
+    - cargo clippy --all-targets --all-features -- -D warnings
     - cargo test
     # etc.
 ```