Add link to custom escaper example in the book

Author: GuillaumeGomez

book/src/configuration.md

@@ -151,4 +151,7 @@ extensions = ["js"]
 [`Escaper`] trait so since we don't need want any escaping on our `.js` files, we use
 it.
 
+You can take a look at the [custom escaper example] in the `askama` repository.
+
 [`Escaper`]: https://docs.rs/askama/latest/askama/filters/trait.Escaper.html
+[custom escaper example]: https://github.com/askama-rs/askama/tree/master/examples/warp-app