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: build the project, then open the .shfbproj file with the installed GUI, click "build the help file", and grab the `.chm` file in `_BuildOutput\Help`. You can then include the file in the Nuget, and publish the generated website on the official site.

This should help in what you mentionned in #85 : providing API documentation.

Note that I branched from the branch build-process (PR #83), but in reality I just added one file.