Add release notes for 0.8.4

Author: MartinOtter

docs/src/index.md

@@ -36,6 +36,10 @@ functionalities of these packages.
 
 ## Release Notes
 
+### Version 0.8.4
+
+- Fix issue with DiffEqBase, version 6.91.6 and later.
+
 ### Version 0.8.3
 
 - Bug fix: Parameters that are Numbers, but not AbstractFloats, and have no unit defined, 
@@ -504,4 +508,4 @@ Changes that are **not backwards compatible** to version 0.7.x:
 
 - [Martin Otter](https://rmc.dlr.de/sr/en/staff/martin.otter/),
   [DLR - Institute of System Dynamics and Control](https://www.dlr.de/sr/en).
-  
+