Add test stub preferences docs

Signed-off-by: Sanjula Ganepola <Sanjula.Ganepola@ibm.com>

Author: SanjulaGanepola

src/content/docs/developing/testing/Testing_TestStubPreferences.png

@@ -49,6 +49,18 @@ After selecting an option, a test suite will be created if it does not already e
 
 ![Generated Test Stub](Testing_GeneratedTestStub.png)
 
+### Test Stub Preferences
+
+There are various preferences that can be used to configure how the test stubs are generated. To access these preferences, go to the extension's settings and search for **Test Stub Preferences**.
+
+![Test Stub Preferences](Testing_TestStubPreferences.png)
+
+### Test Stub Preview
+
+One notable preference is the **Show Test Stub Preview** which controls whether to show a preview of the test stub before adding it to the file or source member. Once this preference is enabled, the **Refactor Preview** view will be populated with a set of edits to insert the control options & directives, includes, prototypes, and the test cases. You will need to first select the checkboxes next to the items you want to insert and then click on any item to bring up a preview of the code in the editor. Once you are satisfied with the preview, you can click the **Apply** button to insert the code into your file or source member.
+
+![Test Stub Preview](Testing_TestStubPreview.png)
+
 ## API Reference
 
 In order to use the APIs provided by RPGUnit, you need to add the `TESTCASE` copybook to your test suite: