Merge pull request #562 from airbnb/ljharb/testing

Start to write a realistic "testing" section

README.md

@@ -2015,14 +2015,22 @@ Other Style Guides
 
 ## Testing
 
-  - [28.1](#28.1) <a name='28.1'></a> **Yup.**
+  - [28.1](#28.1) <a name="28.1"></a> **Yup.**
 
     ```javascript
-    function() {
+    function () {
       return true;
     }
     ```
 
+  - [28.2](#28.2) <a name="28.2"></a> **No, but seriously**:
+   - Whichever testing framework you use, you should be writing tests!
+   - Strive to write many small pure functions, and minimize where mutations occur.
+   - Be cautious about stubs and mocks - they can make your tests more brittle.
+   - We primarily use [`mocha`](https://www.npmjs.com/package/mocha) at Airbnb. [`tape`](https://www.npmjs.com/package/tape) is also used occasionally for small, separate modules.
+   - 100% test coverage is a good goal to strive for, even if it's not always practical to reach it.
+   - Whenever you fix a bug, _write a regression test_. A bug fixed without a regression test is almost certainly going to break again in the future.
+
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