Added doc for slow annotation

docs/index.md

@@ -301,6 +301,7 @@ The following annotations are supported.
 | @noCatch                    | If present, will not catch errors for the test or suite it is placed on. This is handy when developing, and you want to debug the exact line on which an error occurred.                                                                                                                                      | none                                                                                              |
 | @noEarlyExit                | If present, will not exit a test on an assertion failure, which prevents crashes/skewed results. This annotation is mainly used for testing, such as testing rooibos framework itself. It is recommend that you _do not_ use this annotation.                                                                 | none                                                                                              |
 | @async                      | If present, on a test suite or test case (e.g. @it) indicates that the test will execute asynchronously. This will allow you to use observeField in your tests. The only argument is timeout in ms  e.g. @async(2000). Default time out is 2000ms for a test, and 6000 for a test suite                       | max time in ms                                                                                    |
+| @slow                       | Some reporters will display test duration and flag tests that are slow (default: 75ms). This annotation lets you change the default for a single test. e.g. @slow(150)                                                                                                                                        | slow time in ms                                                                                   |