Tweak README

Author: rybakit

README.md

@@ -302,15 +302,16 @@ docker run -d --net=tarantool-php -p 3301:3301 --name=tarantool \
 And then run both unit and integration tests:
 
 ```bash
-docker run --rm --net=tarantool-php -v `pwd`:/queue -w /queue queue
+docker run --rm --net=tarantool-php -v $(pwd):/queue -w /queue queue
 ```
 
-To run only integration or unit tests, set the `PHPUNIT_OPTS` environment variable
-to either `--testsuite integration` or `--testsuite unit` respectively, e.g.:
+The library uses [PHPUnit](https://phpunit.de/) under the hood, and if needed,
+you can pass additional arguments and options to the `phpunit` command.
+For example, to run only unit tests, execute:
 
 ```bash
-docker run --rm --net=tarantool-php -v `pwd`:/queue -w /queue \
-    -e PHPUNIT_OPTS='--testsuite unit' queue
+docker run --rm --net=tarantool-php -v $(pwd):/queue -w /queue \
+    vendor/bin/phpunit --testsuite=unit
 ```