Add note on testInDebug

Addresses point 4 in Issue emberjs#4238

CONTRIBUTING.md

@@ -90,7 +90,7 @@ taken straight from the Ruby on Rails guide:
 * Update the documentation, the surrounding one, examples elsewhere, guides,
   whatever is affected by your contribution
 
-Syntax:
+## Syntax:
 
 * Two spaces, no tabs.
 * No trailing whitespace. Blank lines should not have any space.
@@ -99,4 +99,9 @@ Syntax:
 
 And in case we didn't emphasize it enough: we love tests!
 
+## Writing Tests
+
+* We do write tests for our warns and assertion messages. You may use the `assert.expectAssertion()` and `assert.expectWarning()` helpers for this. Note: Our Travis tests that will run on your PR run in the `production` envrionment, where warnings and assertions are stripped. If you're writing a test for these, you should use `testInDebug` helper rather than `test` to define your test.
+
+
 NOTE: Partially copied from https://raw.github.com/thoughtbot/factory_girl_rails/master/CONTRIBUTING.md