Specify minimum PHPUnit version

ddeboer · ddeboer · commit 2142008c270e · 2015-06-25T21:50:03.000+02:00
diff --git a/doc/contributing.rst b/doc/contributing.rst
@@ -9,6 +9,7 @@ Testing the Library
 -------------------
 
 This chapter describes how to run the tests that are included with this library.
+Make sure that you have PHPUnit 4.0 or higher installed.
 
 First clone the repository, install the vendors, then run the tests:
 
