Merge pull request #3915 from artbristol/diagrams

## Contributing

Thank you for contributing! If you're unfamiliar with the code, 

start by reading the [developer documentation](docs/DESIGN.md)

Please fork this repository and contribute back using [pull requests](https://github.com/k9mail/k-9/pulls).

Any contributions, large or small, major features, bug fixes, unit/integration tests are welcomed and appreciated

# Repository structure

The project is divided into several directories below which are nested gradle projects.

## app

This contains the highest level code such as UI and core logic.

## backend

APIs for sending and receiving messages

## mail

Low level code for dealing with internet mail protocols

## plugins

Additional, standalone, libraries used by K-9

![modules](Modules.png)

# Walkthrough

To help you understand the design, the following sequence diagrams show typical flows through the

classes. Each class is colour-coded by its top-level project. 

## Reading email

![read email sequence](ReadEmail.png)

![read email classes](ReadEmailClasses.png)

## Sending email

![send email sequence](SendEmail.png)