docs(README.md): add timeline to maintenance notice

Author: mboersma

README.md

@@ -1,5 +1,8 @@
-|![](https://upload.wikimedia.org/wikipedia/commons/thumb/1/17/Warning.svg/156px-Warning.svg.png) | Deis Workflow will soon have a [final release](https://deis.com/blog/2017/deis-workflow-final-release/) and go into maintenance mode. After that, this repository will accept critical fixes only until March 1, 2018. |
-|---|---|
+
+|![](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 |
 
 ![](https://deis.com/images/deis-logo.png)
 
@@ -76,3 +79,4 @@ Then view the documentation on [http://localhost:8000](http://localhost:8000) or
 [mkdocs]: http://www.mkdocs.org/
 [issues]: https://github.com/deis/workflow/issues
 [prs]: https://github.com/deis/workflow/pulls
+[v2.18]: https://github.com/deis/workflow/releases/tag/v2.18.0