docs: adds migration guide for refactoring changes

Author: kfc35

release-content/migration-guides/camera_primitives_moved.md

@@ -0,0 +1,30 @@
+---
+title: "Some bevy_camera primitives moved to bevy_math"
+pull_requests: []
+---
+
+`bevy_camera::primitives::HalfSpace` has moved to `bevy_math::primitives::HalfSpace`.
+Some parts of `bevy_camera::primitives::Frustum` has moved to `bevy_math::primitives::ViewFrustum`.
+
+`bevy_camera` has some rendering primitives that can be extracted to be more generally useful.
+To expose them for others to use, some of these primitives and/or functionality have moved to `bevy_math`.
+
+```rust
+// 0.18
+use bevy_camera::primitives::{Frustum, HalfSpace}
+let half_spaces: [HalfSpace; 6] = ...;
+let frustum_one: Frustum = Frustum {
+  half_spaces
+};
+let frustum_two: Frustum = Frustum::from_clip_from_world(...);
+
+// 0.19
+use bevy_math::primitives::{HalfSpace, ViewFrustum}
+use bevy_camera::primitives::Frustum
+let half_spaces: [HalfSpace; 6] = ...;
+let frustum_one: Frustum = Frustum(
+  ViewFrustum {
+    half_spaces
+});
+let frustum_two: Frustum = Frustum(ViewFrustum::from_clip_from_world(...));
+```