Add instructions on how to add modules.

[rl2939]

This ports this Github discussion into a markdown file and turns it into official documentation. It also expands on the additional setup section at the bottom of the original response.

This is a draft pull request because there are two parts in the Troubleshooting section that I don't know or remember the solution to, but have encountered in the past. Feedback on how to solve these issues would help complete the documentation. They are the following:

The module is invisible in the server browser

and

Compiling a module results in the message `fatal error: rack.hpp: No such file or directory`