Document testing locally

I wanted to be able to run the tests locally before submitting a PR, and
didn't see anything. This is a simple addition based on
`.github/workflows/ci.yml` but improvements are welcome!

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+# Contributing
+
+Features and bug fixes are welcome! First-time contributors can jump in with the issues tagged [good first issue](https://github.com/jhedstrom/DrupalDriver/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22).
+
+## Testing
+
+Testing is performed automatically in Github Actions when a PR is submitted. To execute tests locally before submitting a PR, you'll need [Docker and Docker Compose](https://docs.docker.com/engine/install/).
+
+Configure your test environment:
+```
+export PHP_VERSION=8.1
+export DRUPAL_VERSION=10
+export DOCKER_USER_ID=${UID}
+```
+
+Prepare environment for testing:
+```
+docker-compose up -d
+docker-compose exec -T php composer self-update
+docker-compose exec -u ${DOCKER_USER_ID} -T php composer require --no-interaction --dev --no-update drupal/core:^${DRUPAL_VERSION}
+docker-compose exec -T php composer install
+```
+
+Execute all tests:
+```
+docker-compose exec -T php composer test
+```
+
+Execute specific tests, eg just PHPUnit's Drupal7FieldHandlerTest:
+```
+docker-compose exec -T php phpunit --filter Drupal7FieldHandlerTest
+```
+
+- Check the changes from `composer require` are not included in your submitted PR.
+- Before testing another PHP or Drupal version, remove `composer.lock` and `vendor/`

README.md

@@ -65,5 +65,13 @@ $node = (object) array(
 $driver->createNode($node);
 ```
 
+### Contributing
+
+Features and bug fixes are welcome! First-time contributors can jump in with the
+issues tagged [good first issue](https://github.com/jhedstrom/DrupalDriver/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22).
+
+See [CONTRIBUTING.md](https://github.com/jhedstrom/DrupalDriver/blob/master/CONTRIBUTING.md) for more information.
+
 ### Release notes
+
 See [CHANGELOG](CHANGELOG.MD).