Add a test plan for csproj-referencing-xproj scenarios

[jasonmalinowski]

The test plan so far. Still to do:

• [ ] Add some steps for consuming an xproj from a Windows 8.1 Universal app.
• [ ] Add some steps for consuming an xproj from a Xamarin app.

Review: @davkean, @Pilchie, @rchande, @yishaigalatzer, @emgarten

[Pilchie]

I would add the todos directly to the document, even if you don't fill out the sub-bullet instructions for them yet.

[jasonmalinowski]

My intent is to merge with the TODOs done. I just wanted this out for review so far.

[jasonmalinowski]

Per the updated roadmap, this test plan probably doesn't need to exist in this form.