Merge pull request #3976 from vincepri/migration-guide-v1alpha4

📖 Start documenting v1alpha3 to v1alpha4 changes

docs/book/src/developer/providers/v1alpha3-to-v1alpha4.md

@@ -0,0 +1,30 @@
+# Cluster API v1alpha3 compared to v1alpha4
+
+## Minimum Go version
+
+- The Go version used by Cluster API is now Go 1.15+
+
+## Controller Runtime version
+
+- The Controller Runtime version is now v0.7.+
+
+## Kind version
+
+- The KIND version used for this release is v0.9.x
+
+## Upgrade kube-rbac-proxy to v0.5.0
+
+- Find and replace the `kube-rbac-proxy` version (usually the image is `gcr.io/kubebuilder/kube-rbac-proxy`) and update it to `v0.5.0`.
+
+## The controllers.DeleteNodeAnnotation constant has been removed
+
+- This annotation `cluster.k8s.io/delete-machine` was originally deprecated a while ago when we moved our types under the `x-k8s.io` domain.
+
+## The controllers.DeleteMachineAnnotation has been moved to v1alpha4.DeleteMachineAnnotation
+
+- This annotation was previously exported as part of the controllers package, instead this should be a versioned annotation under the api packages.
+
+## Align manager flag names with upstream Kubernetes components
+
+- Rename `--metrics-addr` to `--metrics-bind-addr`
+- Rename `--leader-election` to `--leader-elect`