Update CONTRIBUTING.md

Adding debugging help

CONTRIBUTING.md

@@ -25,6 +25,19 @@ If you don't already have Rust installed rustup is the recommended tool to use.
 
 Then simply clone this project and `cargo build`
 
+### Debugging
+
+You can see the output of tokens by adding a `dbg!(&tokens);` here:
+https://github.com/jasonwilliams/boa/blob/master/src/lib/lib.rs#L31
+
+This is useful to know if the tokens are in the right order, or any unexpected tokens are appearing.
+
+The parser's expression tree can be viewed by adding `dbg!(&expr)` here:
+https://github.com/jasonwilliams/boa/blob/master/src/lib/lib.rs#L34
+
+To get a full backtrace you will need to set the environment variable `RUST_BACKTRACE=1`
+
+
 ### Web Assembly
 
 If you want to develop on the web assembly side you can run yarn serve then go to http://localhost:8080