migration guide for Derive(Event) (#2077)

Co-authored-by: BD103 <[email protected]>

Author: Bleachfuel

release-content/0.16/migration-guides/18083_Improved_DeriveEvent.md

@@ -0,0 +1,16 @@
+In Bevy 0.16 you can now use `#[derive(Event)]` for more specialized implementations. 
+
+```rust
+// 0.15
+struct MyEvent;
+
+impl Event for MyEvent {
+    const AUTO_PROPAGATE: bool = true;
+    type Traversal = &'static ChildOf
+}
+
+// 0.16
+#[derive(Event)]
+#[event(traversal = &'static ChildOf, auto_propagate)]
+struct MyEvent;
+```

release-content/0.16/migration-guides/_guides.toml

@@ -448,6 +448,12 @@ prs = [16139]
 areas = ["ECS", "Reflection"]
 file_name = "16139_ReflectBundleremove_improvement.md"
 
+[[guides]]
+title = "Improved `#[derive(Event)]`"
+prs = [18083]
+areas = ["ECS"]
+file_name = "18083_Improved_DeriveEvent.md"
+
 [[guides]]
 title = "Gamepad improvements"
 prs = [16222]
@@ -1017,4 +1023,4 @@ file_name = "18326_Use_4byte_LightmapSlabIndex_for_batching_instead_of_16byte.md
 title = "Use `target_abi = \"sim\"` instead of `ios_simulator` feature"
 prs = [17702]
 areas = []
-file_name = "17702_Use_target_abi__sim_instead_of_ios_simulator_feature.md"
+file_name = "17702_Use_target_abi__sim_instead_of_ios_simulator_feature.md"