Add particle factory registration event (#6018)

src/main/java/net/minecraftforge/client/event/ParticleFactoryRegisterEvent.java

@@ -0,0 +1,10 @@
+package net.minecraftforge.client.event;
+
+import net.minecraftforge.eventbus.api.Event;
+
+/**
+ * Fired when you should call {@link net.minecraft.client.particle.ParticleManager#registerFactory}.
+ * Note that your {@code ParticleType}s should still be registered during the usual registry events, this
+ * is only for the factories.
+ */
+public class ParticleFactoryRegisterEvent extends Event {}