docs: add link to ASTExplorer in CONTRIBUTING (#186)

Author: trivikr

CONTRIBUTING.md

@@ -29,11 +29,12 @@ Contributions via pull requests are much appreciated. Before sending us a pull r
 To send us a pull request, please:
 
 1. Fork the repository.
-2. Modify the source; please focus on the specific change you are contributing.
-3. Ensure local tests pass.
-4. Commit to your fork using clear commit messages.
-5. Send us a pull request, answering any default questions in the pull request interface.
-6. Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.
+2. Use [ASTExplorer](https://astexplorer.net/) with category `JavaScript` and parser `acorn` for examining the AST.
+3. Modify the source; please focus on the specific change you are contributing.
+4. Ensure local tests pass.
+5. Commit to your fork using clear commit messages.
+6. Send us a pull request, answering any default questions in the pull request interface.
+7. Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.
 
 GitHub provides additional document on [forking a repository](https://help.github.com/articles/fork-a-repo/) and
 [creating a pull request](https://help.github.com/articles/creating-a-pull-request/).