Introduce API annotation.

src/com/sap/piper/API.java

@@ -0,0 +1,19 @@
+package com.sap.piper;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Methods or classes annotated with this annotation are used outside
+ * this shared, e.g. in other shared libraries. In case there is the
+ * need for changing this methods this should be clearly announced
+ * in order to get a consensus about the change and in order to allow
+ * users of the correponding class/method to adapt to the change accordingly.
+ */
+
+@Retention(RetentionPolicy.RUNTIME)
+@Target({ElementType.METHOD, ElementType.TYPE})
+public @interface API {
+}