Document temporary hack

Author: aaemnnosttv

.travis.yml

@@ -32,6 +32,9 @@ install:
   - composer validate --strict
   - composer require --no-update --dev roots/wordpress:${WP_VERSION} wp-phpunit/wp-phpunit:${WP_VERSION}
   - "[ $TRAVIS_PHP_VERSION == '8.0' ] || composer install"
+  # Temporary hack to allow installing PHPUnit 7 with PHP 8 until WP 5.9.
+  # as WP PHPUnit does not allow PHPUnit higher than 7 yet.
+  # See https://github.com/WordPress/wordpress-develop/commit/8def694fe4c5df95f8e20e40389faf9cb92b6dca
   - "[ $TRAVIS_PHP_VERSION != '8.0' ] || composer install --ignore-platform-reqs"
   - composer show