Write a HOWTO on compiling cbt projects with Dotty

[felixmulder]

We need to create some simple documentation on how create a project with dotty using cbt. It should appear on our documentation website: http://lampepfl.github.io/dotty/docs/

• A markdown file should be placed in: https://github.com/lampepfl/dotty/tree/master/docs/docs/usage
• A table of contents entry should be added to: https://github.com/lampepfl/dotty/blob/master/docs/_includes/toc.html#L13

ping: @cvogt

[cvogt]

Happy to add that.

[cvogt]

usage: #1581

[cvogt]

TOC entry: #1582