doc: Add more description of modules

doc/src/04000-modules.md

@@ -9,5 +9,33 @@ linking, tagging or references to files outside of the store or even the store
 interface itself (which by the way shouldn't be used by the end-user at all).
 Others cover things like diary, notes, wiki or bookmarks.
 
-The modules try to offer a consistent commandline user interface.
+We try really hard to offer a consistent commandline user interface over all of
+these modules.
+
+The following sections describe each module in detail, including its purpose and
+its provided backends.
+
+A backend is simply an external tool imag might be able to use.
+For example, the `imag-todo` module offers a `taskwarrior` interface, so imag
+itself does not cover anything which has to do with todo management, but lets
+you continue using `taskwarrior` for that (which does a really good job).
+So what does the `imag-todo` module do?
+Well, it offers you ways to track tasks created in `taskwarrior` and putting
+files which can be used as references to tasks then.
+For example, if you create a task in `taskwarrior`, you end up with an UUID for
+this task.
+imag stores this UUID in a store entry and you are now able to `imag-link` this
+file with other files in the store.
+This way you can link `taskwarrior` tasks with other data (of course,
+`imag-todo` offers some more commands, for searching tasks and so on).
+
+But what if you do not like `taskwarrior`?
+That's what backends are for.
+The goal of imag is to provide backends for not just one tool which implements a
+PIM aspect, but for many.
+So you can change the configuration for `imag-todo` to not use `taskwarrior` but
+some other todo tool.
+
+(This is all hypothetical by now because these things are not yet implemented.
+Anyhow, we aim for exactly what is described above)