add readme.md to every sample

[lucamorelli]

Hi,
I was trying to google for sidewaffle contents and to see if they are easy to discover and I haven't found them, so I think there is a visibility problem.
I suppose that the first way to improve this situation could be to add a readme.md file to every template giving a more detailed description and adding the keywords that could be useful.
I tried to check if readme.md files inside subdirectories are searchable:
first I looked to this sample https://github.com/Microsoft/Windows-universal-samples/tree/master/Samples/XamlMasterDetail, after I googled for "xaml master/detail example" and the page is the first result.
What do you think?