doc: Add definition of modules

doc/src/06000-modules.md

@@ -1,2 +1,20 @@
 # Modules {#sec:modules}
 
+A module is a functionality of the program.
+
+A module MAY store data in the store (@sec:thestore).
+It MAY include user input in the data it stores in the store.
+A module MUST HAVE a commandline interface, though a module always consists of
+two parts:
+
+- A Library
+- A Binary, which
+    * is executable by the user
+    * implements a commandline frontend to the libray of the module
+
+By definition, the Binary depends on the Library.
+By definition, the Library depends on the libstore (@sec:libstore).
+
+A module MUST use the runtime library to implement the commandline
+interface as defined in @sec:librt.
+