added some readme

README.md

@@ -1,4 +1,13 @@
-orca
+Orca
 ====
 
 ![Orca Logo](http://i62.photobucket.com/albums/h100/dallastt/MEME/265899.jpg)
+
+Orca is the orchestration engine for Spinnaker.
+It is responsible for taking a pipeline or task definition and managing the stages and tasks, coordinating the other Spinnaker services.
+
+Orca pipelines are composed of _stages_ which in turn are composed of _tasks_.
+The tasks of a stage share a common context and can publish to a global context shared across the entire pipeline allowing multiple stages to co-ordinate.
+For example a _bake_ stage publishes details of the image it creates which is then used by a _deploy_ stage.
+
+Orca persists a running execution to Redis.