Add a note on MIDI key labels not updating

I think the idea is that normally the plugin will keep copies of the
pointers to the `VstMidiKeyName` structs passed after initialization,
and the host will just read from them whenever it needs to draw these
labels. Since there's no explicit synchronization or notification going
on there's no way for us to tell that these have changed. It would be
possible to implement this by storing these pointers on the plugin side,
storing the objects on the Wine host side, and then periodically polling
for updates. Right now I chose to just not bother with this because it
will add a lot of complexity for very minimal gain (I only noticed it
was an issue after messing with MDrummer and switching between the pad
and loop modes).

README.md

@@ -276,6 +276,10 @@ Aside from that, these are some known caveats:
 - Most recent **iZotope** plugins don't have a functional GUI in a typical out
   of the box Wine setup because of missing dependencies. Please let me know if
   you know which dependencies are needed for these plugins to render correctly.
+- MIDI key labels (for use with drum machines and multisamplers) will not be
+  updated once the plugin has finished loading since there's no way to tell that
+  they have been updated by the plugin. Right now simply deactivating and
+  reactivating the plugin will cause these labels to be updated.
 
 There are also some VST2.X extension features that have not been implemented yet
 because I haven't needed them myself. Let me know if you need any of these