Doc: Add link to project board

Author: VeckoTheGecko

docs/v4/index.md

@@ -4,7 +4,10 @@ This is a central hub to discuss the development of v4. As development progresse
 
 You can think of this as a "living" document as we work towards the release of v4.
 
+Collaboration on v4 development is happening on the [Parcels v4 Project Board](https://github.com/orgs/OceanParcels/projects/5).
+
 ```{toctree}
 
 api
+Parcels v4 Project Board <https://github.com/orgs/OceanParcels/projects/5>
 ```