Update VST readme, for fixing wine 64bit build

Signed-off-by: falkTX <falktx@gmail.com>

Author: falkTX

vstsdk2.4/README

@@ -1 +1,9 @@
 Place the vstsdk2.4 here, namely "pluginterfaces" and "public.sdk" folders.
+
+Note for wine 64bit builds, the file `public.sdk/source/vst2.x/vstplugmain.cpp` needs to be changed, so that a few lines have:
+
+```
+VST_EXPORT VSTCALLBACK AEffect* ...
+```
+
+The `VSTCALLBACK` needs to be added, otherwise it will not work (due to call convention mismatch).