Add MVVM architecture diagram to readme

[jumaallan]

@SanushRadalage

On the project, we don't have the keys.properties file anymore. The configuration moved to gradle.properties, so that setting up Fastlane to read from gradle propeties would be easy

Update the README to reflect the same config setup, but setting the keys on the gradle.properties file - and we have two of these

1. Local one - inside the project (this is commited to git, so not the best place to put the keys)
2. Global one - inside the .gradle folder (this is the preffered one)

Make the change, and the PR will be ready for merge

[codecakes]

@SanushRadalage good to merge