initial cronjob component documentation

doc/components/cronjob.md

@@ -0,0 +1,26 @@
+# Cronjob Component
+
+Should be used when you need to perform a task at a specific time or at regular intervals.
+
+## Usage
+
+The first thing to do is define the task you want to run and add it to a tasks definition map:
+```clojure
+(defn example-task
+  [_as-of
+   {:keys [_components param-test] :as _params}
+   _instance]
+  (log/info ::example-task "Running example task" :param-test param-test))
+
+
+(def tasks {:test-task {:handler  test-task
+                        :schedule "* * * * * * *"
+                        :params   {}}})
+```
+
+After that you need to add the cronjob component to your system map:
+```clojure
+(def system
+  (component/system-map 
+    :cronjob (component.cronjob/new-cronjob <<tasks>>)))
+```