Set up PHPUnit for unit tests

Author: lourot

.gitignore

@@ -1,2 +1,3 @@
 /node_modules/
+/vendor/
 /wpcs/

.prettierignore

@@ -0,0 +1,4 @@
+/node_modules/
+/vendor/
+/wpcs/
+/*.lock

.prettierignore

@@ -5,13 +5,33 @@ pictures!
 
 ## Contributing
 
+### Running the unit tests
+
+#### Setting up PHPUnit
+
+Install [PHPUnit](https://phpunit.readthedocs.io/en/9.5/installation.html)
+with:
+
+```bash
+$ sudo apt install composer
+$ composer install
+```
+
+#### Running PHPUnit
+
+Run the unit tests with:
+
+```bash
+$ composer test
+```
+
 ### Formatting the code
 
 Pull and run [prettier](https://github.com/prettier/plugin-php) with:
 
 ```bash
-$ yarn install
-$ yarn run prettier --write .
+$ yarn install     # pull
+$ composer format  # run
 ```
 
 ### Validating against WordPress coding standards
@@ -64,5 +84,5 @@ $ popd
 Run PHP_CodeSniffer with:
 
 ```bash
-$ ./wpcs/vendor/bin/phpcs -s
+$ composer lint
 ```

README.md

@@ -0,0 +1,10 @@
+{
+  "require-dev": {
+    "phpunit/phpunit": "^9.5.25"
+  },
+  "scripts": {
+    "format": "yarn run prettier --write . || yarnpkg run prettier --write .",
+    "lint": "./wpcs/vendor/bin/phpcs -s",
+    "test": "./vendor/bin/phpunit"
+  }
+}