[Documentation] Add comments for important/public functions

## Problem
Sometimes a function name alone is not enough to describe what a function does. Adding documentation will improve the quality of the project and ease of use.

## Feature Description
Add comments for important private functions and all public functions

## Alternatives
Not adding documentation. (Everybody is unhappy, including the linter.)

## Additional Context
While the project is small, it should be straight forward, most important thing is to follow golang conventions. (https://blog.golang.org/godoc-documenting-go-code)