Document json patches usage

Signed-off-by: alexander-demicev <[email protected]>

docs/README.md

@@ -754,3 +754,32 @@ spec:
   additionalManifests:
     name: additional-manifests
 ```
+
+## Patching provider manifests
+
+Provider manifests can be patched using JSON merge patches. This can be useful when you need to modify the provider manifests that are fetched from the repository.
+
+```yaml
+---
+apiVersion: operator.cluster.x-k8s.io/v1alpha2
+kind: CoreProvider
+metadata:
+  name: cluster-api
+  namespace: capi-system
+spec:
+  resourcePatches:
+    - |
+apiVersion: v1
+kind: Service
+metadata:
+labels:
+    test-label: test-value
+```
+
+There couple rules for the patch to match a manifest:
+
+- The `kind` field must match the target object.
+- If `apiVersion` is specified it will only be applied to matching objects.
+- If `metadata.name` and `metadata.namespace` not specified, the patch will be applied to all objects of the specified kind.
+- If `metadata.name` is specified, the patch will be applied to the object with the specified name. This is for cluster scoped objects.
+- If both `metadata.name` and `metadata.namespace` are specified, the patch will be applied to the object with the specified name and namespace.