Builds help files

I've added a [SandCastle Helpfile Builder](https://shfb.codeplex.com/) file to build documentation.

Unfortunately, I couldn't find a way to automatically build the file in a separate server, since it requires a bunch of SDKs and stuff to be installed. So you'll have to install SHFB on your PC, and generate the documentation manually.

This should help in what you mentionned in #85 : providing API documentation.