Add troubleshooting information for tools (e.g. MsBuild)

[pascalberger]

We regularly get questions why running a specific tool (e.g. MsBuild) doesn't work with Cake. We should add a troubleshooting section or FAQ entry explaining that Cake only shells out to the external tool and that e.g. getting in to run first on command line can help (also with information how to see what command line Cake calls).