Documentation of the external API.

[skyem123]

This is very important for a library, and it will also be good to have tutorials in this repo.

_Note that this issue will not be closed, as it is an important reminder to new code written_

[skyem123]

Internal documentation is useful, but not needed, so it will go into issue #13 .
Documentation of the external API is important, so that is what this issue will focus on.

[skyem123]

Sadly, KDoc doesn't compile for me, and Dokka requires extra setup on the computer.
However, I can do some markdown documentation in this repository.