Documentation for tests that rely on working internet connection

Author: clue

README.md

@@ -131,6 +131,14 @@ To run the test suite, go to the project root and run:
 $ php vendor/bin/phpunit
 ```
 
+The test suite also contains a number of functional integration tests that send
+test HTTP requests against the online service http://httpbin.org and thus rely
+on a stable internet connection.
+If you do not want to run these, they can simply be skipped like this:
+
+```bash
+$ php vendor/bin/phpunit --exclude-group internet
+
 ## License
 
 MIT, see [LICENSE file](LICENSE).