Feature: add roadmap to README

[achille-martin]

According to https://github.com/jonashackt/plantuml-markdown, it is possible to add a plantuml diagram to Github readme:

• Create file .iuml
• Add to github
• Use raw link to display -> ! [your-UML-diagram-name] (http://www.plantuml.com/plantuml/proxy?cache=no&src=https://raw.githubusercontent.com/jonashackt/plantuml-markdown/master/example-uml.iuml)

[achille-martin]

The file .iuml can be added to documentation-resources as roadmap.iuml with the content:

@startuml
start
partition Overview {
    :Articulate aim;
    note right
        Create an android app (.apk) from a simple PyQt5 app
    end note
    :Define requirements;
    note right
      Relevant for custom apps
    end note
    :Research existing projects;
    note right
        **Pyqtdeploy** / Pyside / Kivy / BeeWare
    end note
}
partition Implementation {
    :Apk generation;
    note right
      Managed by build_app.py
    end note
    :Pkg structure;
    note right
      Basic python project structure
    end note
    :SQL database;
    note right
      Managed with QtSql
    end note
    :Graphics;
    note right
      WIP
    end note    
}
partition Testing {
    :Logging;
    note right
      Basic python logging
    end note
    :Tests;
}
partition Deployment {
    :Documentation;
    :Sharing and Advertising;
}
partition Extension {
    :Windows OS;
    :Linux OS;
    :Mac OS;
}
stop
@enduml

[achille-martin]

Inside the README:

<a id="roadmap"></a>
## Roadmap

_This section introduces the broad roadmap for this project._

![Roadmap Diagram](http://www.plantuml.com/plantuml/proxy?cache=no&src=https://raw.githubusercontent.com/achille-martin/simple-pyqt5-android-app/master/documentation-resources/roadmap.iuml)