Add style guide

CONTRIBUTING.md

@@ -20,17 +20,95 @@ Make sure that every file that contains significant work (at least every code fi
 starts with the copyright header:
 
 ```
-/***************************************************************************************************
+/*
  * Copyright © All Contributors. See LICENSE and AUTHORS in the root directory for details.
- **************************************************************************************************/
+ */
 ```
 
 You can set this in Android Studio:
 
 1. Settings / Editor / Copyright / Copyright Profiles
 2. Paste the text above (without the stars).
-3. Set Formatting: separator before and after, length: 100.
+3. Set Formatting so that the preview exactly looks like above; one blank line after the block.
 4. Set this copyright profile as the default profile for the project.
+5. Apply copyright: right-click in file tree / Update copyright.
+
+
+# Style guide
+
+Please adhere to the [Kotlin style guide](https://developer.android.com/kotlin/style-guide) and
+the following hints to make the source code uniform.
+
+**Have a look at similar files and copy their style if you're not certain.**
+
+Sample file (pay attention to blank lines and other formatting):
+
+```
+<Copyright header, see above>
+
+class MyClass(int arg1) : SuperClass() {
+
+    companion object {
+
+        const val CONSTANT_STRING = "Constant String";
+
+        fun staticMethod() {	// Use static methods when you don't need the object context.
+            // …
+        }
+
+    }
+
+    var someProperty: String = "12345"
+    var someRelatedProperty: Int = 12345
+
+    init {
+        // constructor
+    }
+
+
+    /**
+     * Use KDoc to document important methods. Don't use it dogmatically, but writing proper documentation
+     * (not just the method name with spaces) helps you to re-think what the method shall really do.
+     */
+    fun aFun1() {		// Group methods by some logic (for instance, the order in which they will be called)
+    }				// and alphabetically within a group.
+
+    fun anotherFun() {
+        // …
+    }
+
+
+    fun somethingCompletelyDifferent() {	// two blank lines to separate groups
+    }
+
+    fun helperForSomethingCompletelyDifferent() {
+        someCall(arg1, arg2, arg3, arg4)	// function calls: stick to one line unless it becomes confusing
+    }
+
+
+    class Model(				// two blank lines before inner classes
+        someArgument: SomeLongClass,		// arguments in multiple lines when they're too long for one line
+        anotherArgument: AnotherLongType,
+        thirdArgument: AnotherLongTypeName
+    ) : ViewModel() {
+
+        fun abc() {
+        }
+
+    }
+
+}
+```
+
+In general, use one blank line to separate things within one group of things, and two blank lines
+to separate groups. In rare cases, when methods are tightly coupled and are only helpers for another
+method, they may follow the calling method without separating blank lines.
+
+## Tests
+
+Test classes should be in the appropriate directory (see existing tests) and in the same package as the
+tested class. Tests are usually be named like `methodToBeTested_Condition()`, see
+[Test apps on Android](https://developer.android.com/training/testing/).
 
 
 # Authors