Add topics (Github new feature like tag/category)

[peterblazejewicz]

Topics are labels that create subject-based connections between GitHub repositories and let you explore projects by type, technology, and more.

https://github.com/blog/2309-introducing-topics

This requires project owner: @sayedihashimi

Thanks!

[sayedihashimi]

We are shutting this repo down, it's been replaced with dotnet new.