Add base help for preview command

Author: Fatme

docs/man_pages/project/testing/preview.md

@@ -0,0 +1,15 @@
+<% if (isJekyll) { %>---
+title: tns preview
+position: 1
+---<% } %>
+# tns preview
+
+
+Usage | Synopsis
+---|---
+Produces a QR code for deployment in the Preview app. | `tns preview`
+
+Enjoy NativeScript without any local setup. All you need is a couple of companion apps installed on your devices. When you save changes to the project, the changes are automatically synchronized to Preview app on your device.
+
+### Options
+`--bundle` - Specifies that a bundler (e.g. webpack) should be used if one is present. If no value is passed will default to `webpack`