add tests advice

Author: Minishlink

CONTRIBUTING.md

@@ -1,3 +1,27 @@
 # Contributing to WebPush
 WebPush is an open source library.
 Feel free to contribute by submitting a pull request or creating (and solving) issues!
+
+## Running Tests
+
+First, you will need to create your own configuration file by copying
+phpunit.dist.xml to phpunit.xml and filling in the fields you need for
+testing (i.e. STANDARD_ENDPOINT, GCM_API_KEY etc.).
+
+Then, download [phpunit](https://phpunit.de/) and test with one of the
+following commands:
+
+**For All Tests**
+    `php phpunit.phar`
+
+**For a Specific Test File**
+    `php phpunit.phar tests/EncryptionTest.php`
+
+**For a Single Test**
+    `php phpunit.phar . --filter "/::testPadPayload( .*)?$/"` (regex)
+
+Some tests have a custom decorator @skipIfTravis. The reason is that
+there's no way in Travis to update the push subscription, so the endpoint
+in my phpunit.travis.xml would ultimately expire
+(and require a human modification), and the corresponding tests would fail.
+But locally, these tests are handy.

README.md

@@ -230,8 +230,5 @@ This library was inspired by the Node.js [marco-c/web-push](https://github.com/m
 ## Contributing
 See [CONTRIBUTING.md](https://github.com/Minishlink/web-push/blob/master/CONTRIBUTING.md).
 
-## Tests
-Copy `phpunit.xml` from `phpunit.dist.xml` and fill it with your test endpoints and private keys.
-
 ## License
 [MIT](https://github.com/Minishlink/web-push/blob/master/LICENSE)