README: Link to 6.x upgrade guide

Author: angularsen

README.md

@@ -8,7 +8,8 @@ Add strongly typed quantities to your code and get merrily on with your life.
 
 No more magic constants found on Stack Overflow, no more second-guessing the unit of parameters and variables.
 
-[Upgrading from 4.x to 5.x](https://github.com/angularsen/UnitsNet/wiki/Upgrading-from-4.x-to-5.x)
+[Upgrading from 4.x to 5.x](https://github.com/angularsen/UnitsNet/wiki/Upgrading-from-4.x-to-5.x)<br>
+[Upgrading from 5.x to 6.x](https://github.com/angularsen/UnitsNet/wiki/Upgrading-from-5.x-to-6.x) (still in prerelease and may yet change)
 
 ### Overview