add roadmap section to docs (#432)

docs/_src/usage/usage/roadmap.md

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+<!---
+title: "Roadmap"
+metaTitle: "Roadmap and Zenhub Access"
+metaDescription: "How to access our open roadmap"
+slug: "/docs/roadmap"
+date: "2020-09-24"
+id: "roadmapmd"
+--->
+
+# Open roadmap
+
+We believe open-source is more than open source code. It's a lot about people, collaboration, transparency and trust. 
+Therefore, we decided to be as open as possible with our roadmap and sprint planning. 
+In fact, you can see all of it in real-time on GitHub. 
+We hope this helps to clarify the direction of the open-source project and inspires discussions in the community. 
+
+## How to access it
+
+### Add Zenhub Plugin
+We decided for Zenhub, as it allows a close integration with GitHub and real-time sharing of roadmaps and sprints.
+Once you have installed the browser plugin from here, you will see additional tabs and infos on the Haystack GtiHub page.
+
+Zenhub Plugin: https://www.zenhub.com/extension 
+
+### Roadmap
+_Zenhub Tab -> Roadmap (right menu)_
+
+Here you can find our most recent roadmap with the **high-level projects** that are planned for the upcoming quarters.
+We update it regularly and refine the projects as they come closer in the time line. 
+ 
+![image](../../img/zenhub_roadmap.png)
+
+### Board
+_Zenhub Tab -> Board (right menu)_
+
+If you are interesting in the **operational tasks** and their status, you can find our agile board here. 
+
+![image](../../img/zenhub_board.png)
+### Additional issue details & Releases
+_Right panel in regular Github issues_
+
+With Zenhub you can also see some additional tags in every GitHub issue. 
+For those of you who wonder about the next release date: We aim for releases every ~ 4 weeks and will tag the issues that will need to be finished before we can release a bit in advance. 
+
+![image](../../img/zenhub_issue.png)