docs: document vendored_phpunit_path

Author: Greg Bowler

README.md

@@ -47,6 +47,7 @@ The following configuration options are available:
 + `version` The version of PHPUnit to use e.g. `9` or `9.5.0` (default: latest)
 + `php_version` The version of PHP to use e.g. `7.4` (default: latest)
 + `php_extensions` Space-separated list of extensions using [php-build][php-build] e.g. `xdebug mbstring` (default: N/A)
++ `vendored_phpunit_path` The path to a phar file already present on the runner (default: N/A)
 + `configuration` Path to the `phpunit.xml` file (default: `test/phpunit/phpunit.xml`)
 + `log_junit` Path to junit output file (default: `test/phpunit/_junit/junit.xml`)
 + `memory_limit` The memory limit to run your tests with (default: `128M`)