chore: update quick iteration section in contributing guide

remove references to `lr` and update them with `yarn` instead.

This section will be an entry point to contributors who will not have set up their aliases.
It will also become an entry point for the new gitpod flow

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

CONTRIBUTING.md

@@ -508,27 +508,35 @@ The `dist/` folder within each module contains the packaged up language artifact
 
 ### Quick Iteration
 
-After you've built the modules you want to work on once, use `lr watch` for each module that you are modifying.
+After you've built the modules you want to work on once, use `yarn watch` for each module that you are modifying.
 
 Watch the EC2 and IAM modules in a second terminal session:
 
 ```console
 $ cd packages/@aws-cdk/aws-ec2
-$ lr watch & # runs in the background
+$ yarn watch & # runs in the background
 $ cd packages/@aws-cdk/aws-iam
-$ lr watch & # runs in the background
+$ yarn watch & # runs in the background
 ```
 
 Code...
 
-Now to test, you can either use `lr test` or invoke nodeunit directory (faster, since "test" will also build):
+Now to test, you can either use `yarn test` or invoke nodeunit/jest directly:
 
+Running nodeunit tests directly on a module
 ```console
 $ cd packages/@aws-cdk/aws-iam
 $ nodeunit test/test.*.js
 <BOOM>
 ```
 
+Running jest tests directly on a module
+```console
+$ cd packages/@aws-cdk/aws-iam
+$ jest test/*test.js
+<BOOM>
+```
+
 ### Linking against this repository
 
 The script `./link-all.sh` can be used to generate symlinks to all modules in this repository under some `node_module`