Add CONTRIBUTING.md and/or comments

[pludi]

Currently there is no information as to how to setup a development environment for this, nor any description in the source on how things work or how settings affect anything. For example in app/src/main/java/com/svenjacobs/app/leon/domain/model/Sanitizer.kt on how the Regex has to be constructed to pull the required information, or what isDefault or uid are used for.

Since this is a rather new and small app this could help bring in developers just starting with Android development.

[svenjacobs]

Hi @pludi, thanks for your feedback. You're right, I certainly need to improve code documentation 😅