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 Builder v2
 
 [![Build Status](https://ci.deis.io/job/builder/badge/icon)](https://ci.deis.io/job/builder) [![codecov](https://codecov.io/gh/deis/builder/branch/master/graph/badge.svg)](https://codecov.io/gh/deis/builder)
@@ -78,3 +84,4 @@ Please follow the instructions on the [official Deis docs](http://docs-v2.readth
 [k8s-home]: http://kubernetes.io
 [issues]: https://github.com/deis/builder/issues
 [prs]: https://github.com/deis/builder/pulls
+[v2.18]: https://github.com/deis/workflow/releases/tag/v2.18.0