Amending commands to install the testing frameworks

If you want to add testing to an already working project, the search for the correct plugins can be tedious, thus the request to add this in the "getting started" paragraph.

Author: lacrioque

docs/guides/getting-started.md

@@ -10,6 +10,22 @@ cd vue-test-utils-getting-started
 npm install
 ```
 
+If you already have a project and want to add testing capabilities you may run:
+
+```bash
+# unit testing
+vue add @vue/unit-jest 
+
+#or:
+vue add @vue/unit-mocha
+
+# end-to-end
+vue add @vue/e2e-cypress 
+
+#or:
+vue add @vue/e2e-nightwatch
+```
+
 You will see that the project includes a simple component, `counter.js`:
 
 ```js