Adds first version of developer docs, dealing with build flags

DEVELOPMENT.md

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+# Development
+
+Gauguin uses, depends, builds on
+
+* OS: Android
+* language: Kotlin
+* build system: Gradle
+* UI: Material via XML (switching to Compose is a long term thing)
+
+## Gradle modules
+
+There are two modules to separate the core logic from Android specific and/or UI specific code:
+
+* gauguin-core: Plain Kotlin module without any Android or ui related dependencies.
+* gauguin-app: Android app module using gauguin-core, Android SDK, AndroidX, Material Design
+Components.
+
+## Build flags
+
+These flags are passed to Gradle and are interpreted by custom Kotlin code included in Gradle Kotlin
+DSL. To set this flag via Gradle wrapper, use e.g. for flag `buildserver`
+
+    ./gradlew build -Pbuildserver
+
+All flags default to false and may be activated to switch them to `true`. You can combine multiple
+flags like this:
+
+    ./gradlew build -Pbuildserver -Pscreenshot
+
+### Flag buildserver
+
+The build flag `buildserver` lets some tasks run long running stuff beeing used in CI builds only.
+This triggers the following
+
+| Value | Behaviour                                                                |
+|-------|--------------------------------------------------------------------------|
+| false | Deactivate Sonar as whole.                                               |
+| true  | Activate Sonar, let Sonar depend on Android linter results (task `lint`) |
+
+### Screenshot tests
+
+`screenshot` needs to be set to activate screenshot tests. Without this flag, no screenshot test
+will be run as they need quite some time.