Add exploratory column to readme

Author: alexcnichols

README.md

@@ -41,7 +41,7 @@ Feature under consideration. We are considering building this feature, and gathe
 
 ## Roadmap stages
 
-The roadmap is arranged on a project board to give a sense for how far out each item is on the horizon. Every product or feature is added to a particular project board column according to the quarter in which it is expected to ship next. Be sure to read the [disclaimer](#disclaimer) below since the roadmap is subject to change, especially further out on the timeline.
+The roadmap is arranged on a project board to give a sense for how far out each item is on the horizon. Every product or feature is added to a particular project board column according to the quarter in which it is expected to ship next. Be sure to read the [disclaimer](#disclaimer) below since the roadmap is subject to change, especially further out on the timeline.  You'll also find an **Exploratory** column, which is used in conjunction with the **in design** and **exploring** release phase labels for when no timeframe is yet available.
 
 GitHub Enterprise Server has major releases on a quarterly basis, and minor releases on a monthly basis. Once we know what version we are delivering a feature, we will update the issue to indicate that information.