CONTRIBUTING: add minimal info about bashunit tests

Author: jrfnl

.github/CONTRIBUTING.md

@@ -18,7 +18,8 @@ Thank you for your interest in contributing to PHP_CodeSniffer!
     * [Finding Something to Work on](#finding-something-to-work-on)
     * [Getting Started](#getting-started)
     * [While Working on a Patch](#while-working-on-a-patch)
-    * [Writing Tests](#writing-tests)
+    * [Writing Unit/Integration Tests](#writing-unitintegration-tests)
+    * [Writing End-to-End Tests](#writing-end-to-end-tests)
     * [Submitting Your Pull Request](#submitting-your-pull-request)
 * [Licensing](#licensing)
 
@@ -268,7 +269,7 @@ To help you with this, a number of convenience scripts are available:
 N.B.: You can ignore any skipped tests as these are for external tools.
 
 
-### Writing Tests
+### Writing Unit/Integration Tests
 
 Tests for the PHP_CodeSniffer engine can be found in the `tests/Core` directory.
 Tests for individual sniffs can be found in the `src/Standards/[StandardName]/Tests/[Category]/` directory.
@@ -376,7 +377,7 @@ To run the tests specific to the use of `PHP_CODESNIFFER_CBF === true`:
     vendor/bin/phpunit --group CBF --exclude-group nothing
     ```
 
-#### Other notes about writing tests
+#### Other notes about writing unit/integration tests
 
 * The `Config` class uses a number of static properties and can have a performance impact on the tests too.
     To get round both these issues, use the `ConfigDouble` class instead.
@@ -387,6 +388,24 @@ To run the tests specific to the use of `PHP_CODESNIFFER_CBF === true`:
 * When using data providers, define them immediately below the corresponding test method.
 * When a test method has only one data provider, it is considered best practice to closely couple the test and data provider methods via their names. I.e. the data provider's name should match the test method name, replacing the "test" prefix with "data". For example, the data provider for a method named `testSomething()` should be `dataSomething()`.
 
+
+### Writing End-to-End Tests
+
+Bash-based end-to-end tests can be written using the [Bashunit](https://bashunit.typeddevs.com/) test tooling.
+
+To install bashunit, follow the [installation guide](https://bashunit.typeddevs.com/installation).
+
+You can then run the bashunit tests on Linux/Mac/WSL, like so:
+```bash
+./lib/bashunit -p tests/EndToEnd
+```
+
+> Note: these tests will not run in the Windows native CMD shell. When on Windows, either use WSL or use the "git bash" shell.
+
+When writing end-to-end tests, please use fixtures for the "files under scan" to make the tests stable.
+These fixtures can be placed in the `tests/EndToEnd/Fixtures` subdirectory.
+
+
 ### Submitting Your Pull Request
 
 Some guidelines for submitting pull requests (PRs) and improving the chance that your PR will be merged:

.gitignore

@@ -9,3 +9,4 @@ composer.lock
 phpstan.neon
 /node_modules/
 /tests/EndToEnd/Fixtures/*.fixed
+/lib/