Add migration guide

Author: Jondolf

migration-guides/0.6-to-main.md

@@ -16,3 +16,14 @@ simply ignore the new parameter with `_`.
 
 Similar to many other plugins, the `SpatialQueryPlugin` now stores a schedule label,
 which by default is also passed via `PhysicsPlugins::new`.
+
+## Physics Debug Gizmo Mesh Visibility
+
+In past releases, `PhysicsGizmos::hide_meshes` forced all meshes to be visible when it is `false`.
+Im **Avian 0.7**, it instead sets the mesh visibility to `Visibility::Inherited`, which is a more
+appropriate default.
+
+This is still not perfect, as it does not fully respect the old visibility value. Doing so
+would require caching the old visibility in a separate component. This feature will likely
+undergo larger changes in the future, for example favoring `RenderLayers` over `Visibility`
+to create a dedicated "debug view".