docs: add Testing section to CONTRIBUTING.md

CONTRIBUTING.md

@@ -46,6 +46,50 @@ started:
 👩‍⚖️ When contributing code to this project, you may be asked to agree to our
 [Contributor License Agreement].
 
+## Testing
+
+The [devcontainer CLI] is used to run the tests in this repository. Install it with:
+
+```bash
+npm install -g @devcontainers/cli
+```
+
+### Run all tests
+
+To run the tests for **every** Feature in this repository, run the following commands from the root of the repository:
+
+```bash
+# Run auto-generated and scenario tests for every Feature
+devcontainer features test .
+
+# Run the global test scenarios
+devcontainer features test --global-scenarios-only .
+```
+
+### Test a specific Feature
+
+To test a specific Feature (e.g. `go`):
+
+```bash
+devcontainer features test -f go .
+```
+
+### Test against a specific base image
+
+By default tests run against `mcr.microsoft.com/devcontainers/base:ubuntu`. Use the `--base-image` (or `-i`) flag to specify a different image:
+
+```bash
+devcontainer features test -f go -i ubuntu:focal .
+```
+
+### Run only scenario tests
+
+Use `--skip-autogenerated` to run only the [scenario tests] defined in `test/<feature>/scenarios.json`:
+
+```bash
+devcontainer features test -f go --skip-autogenerated .
+```
+
 <!-- prettier-ignore-start -->
 [our dev container community Slack channel]: https://aka.ms/devcontainer_community
 [open an issue]: https://github.com/devcontainers/features/issues/new
@@ -57,4 +101,6 @@ started:
 [contributor license agreement]: https://opensource.microsoft.com/cla/
 [Test your changes using `devcontainer features test`]: https://github.com/devcontainers/cli/blob/main/docs/features/test.md
 [semver]: https://semver.org/
+[devcontainer CLI]: https://github.com/devcontainers/cli
+[scenario tests]: https://github.com/devcontainers/cli/blob/main/docs/features/test.md#scenarios
 <!-- prettier-ignore-end -->