Update CONTRIBUTING.md

Author: tianchu

CONTRIBUTING.md

@@ -2,15 +2,20 @@
 
 We love pull requests. Here's a quick guide.
 
-Fork, then clone the repo:
+Fork, then clone:
 
-    git clone git@github.com:DataDog/datadog-lambda-layer-python.git
+```bash
+git clone git@github.com:your-username/datadog-lambda-layer-python.git
+```
 
-Make your change. Add tests for your change. Make the tests pass again.
+Make your change, update tests and ensure the tests pass (install Docker if you haven't):
+
+```bash
+./scripts/run_tests.sh
+```
 
 Push to your fork and [submit a pull request][pr].
 
 [pr]: https://github.com/your-username/datadog-lambda-layer-python/compare/DataDog:master...master
 
 At this point you're waiting on us. We may suggest some changes or improvements or alternatives.
-