docs(README.md): add notice of final release and maintenance mode

Author: mboersma

README.md

@@ -1,3 +1,9 @@
+
+|![](https://upload.wikimedia.org/wikipedia/commons/thumb/1/17/Warning.svg/156px-Warning.svg.png) | Deis Workflow will soon no longer be maintained.<br />Please [read the announcement](https://deis.com/blog/2017/deis-workflow-final-release/) for more detail. |
+|---:|---|
+| 09/07/2017 | Deis Workflow [v2.18][] final release before entering maintenance mode |
+| 03/01/2018 | End of Workflow maintenance: critical patches no longer merged |
+
 # Deis Client
 
 [![Build Status](https://ci.deis.io/buildStatus/icon?job=Deis/workflow-cli/master)](https://ci.deis.io/job/Deis/job/workflow-cli/job/master/)
@@ -149,3 +155,5 @@ To learn more about a command run `deis help <command>`.
 ## License
 
 see [LICENSE](https://github.com/deis/workflow-cli/blob/master/LICENSE)
+
+[v2.18]: https://github.com/deis/workflow/releases/tag/v2.18.0