Update the README Issues (#350)

We've had these for a while now:
 - D/I cache
 - GPR bypassing
 - Supervisor state (and can boot linux)

We still need Vector/VMX/VSX (and probably some other things)

Signed-off-by: Michael Neuling <mikey@neuling.org>

README.md

@@ -139,8 +139,5 @@ make -j$(nproc) check
 
 ## Issues
 
-This is functional, but very simple. We still have quite a lot to do:
-
-- There are a few instructions still to be implemented
-- Need to add caches and bypassing (in progress)
-- Need to add supervisor state (in progress)
+- There are a few instructions still to be implemented:
+  - Vector/VMX/VSX