Add @UnstableApi annotation. Resolves #313

api/src/main/java/com/velocitypowered/api/annotations/UnstableApi.java

@@ -0,0 +1,14 @@
+package com.velocitypowered.api.annotations;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Target;
+
+/**
+ * Marks unstable API interfaces that are still maturing. These interfaces may change drastically
+ * between minor releases of Velocity, and it is not guaranteed that the APIs marked with this
+ * annotation will be stable over time.
+ */
+@Target({ ElementType.METHOD, ElementType.TYPE, ElementType.PACKAGE })
+public @interface UnstableApi {
+
+}