CB-3356-Clarify the version number used in screen cap

Hello, I received a report that users were getting confused about the cordova version number used in the screen captures in the WP8 Getting Started guide. I added a small disclaimer note like the ones used in the iOS Getting Started guide. 

For CB-3356. I feel like something this small doesn't need a JIRA item but based on the recent discussion on the ML I went ahead and created it anyway.