update README

linux/include/polyvox_filament/polyvox_filament_plugin.h

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+#ifndef FLUTTER_PLUGIN_POLYVOX_FILAMENT_PLUGIN_H_
+#define FLUTTER_PLUGIN_POLYVOX_FILAMENT_PLUGIN_H_
+
+#include <flutter_linux/flutter_linux.h>
+
+G_BEGIN_DECLS
+
+#ifdef FLUTTER_PLUGIN_IMPL
+#define FLUTTER_PLUGIN_EXPORT __attribute__((visibility("default")))
+#else
+#define FLUTTER_PLUGIN_EXPORT
+#endif
+
+typedef struct _PolyvoxFilamentPlugin PolyvoxFilamentPlugin;
+typedef struct {
+  GObjectClass parent_class;
+} PolyvoxFilamentPluginClass;
+
+FLUTTER_PLUGIN_EXPORT GType polyvox_filament_plugin_get_type();
+
+FLUTTER_PLUGIN_EXPORT void polyvox_filament_plugin_register_with_registrar(
+    FlPluginRegistrar* registrar);
+
+G_END_DECLS
+
+#endif  // FLUTTER_PLUGIN_POLYVOX_FILAMENT_PLUGIN_H_