docs: mention Bulloak in contributing doc

CONTRIBUTING.md

@@ -46,6 +46,10 @@ $ bun run
 When making a pull request, ensure that:
 
 - All tests pass.
+- Concrete tests are generated using Bulloak and the Branching Tree Technique (BTT).
+  - You can learn more about this on the [Bulloak website](https://bulloak.dev).
+  - If you modify a test tree, use this command to generate the corresponding test contract that complies with BTT:
+    `bulloak scaffold -wf /path/to/file.tree`
 - Code coverage remains the same or greater.
 - All new code adheres to the style guide:
   - All lint checks pass.