Update the issue template to tell you where to find command callback tests

PULL_REQUEST_TEMPLATE.md

@@ -12,7 +12,8 @@ READ THIS: Before creating a pull request, please consider the following first.
   that ALE can work with, please add Vader tests for them. Look at existing
   tests in the test/handler directory, etc.
 * If you add or modify a function for computing a command line string for
-  running a command, please add Vader tests for that.
+  running a command, please add Vader tests for that. Look at existing
+  tests in the test/command_callback directory, etc.
 * Generally try and cover anything with Vader tests, although some things just
   can't be tested with Vader, or at least they can be hard to test. Consider
   breaking up your code so that some parts can be tested, and generally open up