Add C# documentation

Sorry to bother you again, but I just had a look at the visual diff and this is rendered incorrectly. It confuses the hash as end-of-header marker and just omits it. Unfortunately escaping with `\#` doesn't work either, which is a kramdown [bug](https://github.com/gettalong/kramdown/issues/164) I reported a while ago, but GitHub Pages uses an older version so we're out of luck.

The same happens for F# as well. As a workaround, we could collapse those three sections into one "C#, F# and Visual Basic" so there's no # at the end. What do you think?