begin to document cache

Signed-off-by: Nico Schottelius <nico@kr.ethz.ch>

doc/dev/todo/niconext

@@ -1,27 +1,6 @@
-Core:
-   - support $__self = relative_type/object_id
-
 Cache:
-   - add manpage
    - add example how to use 
    - export variable $__cache
       -> to all hosts or this host?
-
-   Assume you want to configure stuff one host ("monitor node"),
-   depending on the configuration of other hosts ("cluster nodes").
-
-   For instance, the monitor host would like to know,
-   which hosts are configured with the provider
-   "apache" and option --start true.
-
-   This requires the monitor node to be able to
-   query all other configured nodes. It can't
-   ask for all hosts, because cdist does not
-   know which hosts are configured or may exist.
-
-   Example implementation
-
-   If cdist keeps ("caches") the configuration of every
-   node it configures, each new node can query the
-   cache for existing nodes that acquired the given
-   configuration.
+Core:
+   - support $__self = relative_type/object_id