Added a KB style document about RestMapper (#462)

docs/RestMapper.md

@@ -0,0 +1,61 @@
+# RestMapper
+
+## Using the RestMapper in a reconciler.
+
+It's sometimes necessary to access the RestMapper in your reconciler. The RestMapper's
+purpose is to map between the `R` (Resource) in `GVR` and the  `K` (Kind) in `GVK`.
+
+Unfortunately `rtesting` (Reconciler-Runtime's [testing](../README.md#testing)) cannot 
+populate the RestMapper automatically. The information for this mapping is (typically)
+generated by controller-gen and placed into manifests.
+
+For RestMapper to work, you will need to populate the RestMapper with the GVK you want
+it to resolve.
+
+### Example: Using the resource guesser
+
+[DefaultRESTMapper.Add](https://pkg.go.dev/k8s.io/apimachinery/pkg/api/meta@v0.28.4#DefaultRESTMapper.Add) adds the GVK 
+with a guessed singular and plural resource name. (See [UnsafeGuessKindToResource](https://pkg.go.dev/k8s.io/apimachinery/pkg/api/meta@v0.28.4#UnsafeGuessKindToResource)
+for how this is done)
+
+```
+	rts.Run(t, scheme, func(t *testing.T, rtc *rtesting.ReconcilerTestCase, c reconcilers.Config) reconcile.Reconciler {
+		gvk := schema.GroupVersionKind{
+			Group:   "example.com",
+			Version: "v1alpha1",
+			Kind:    "Widget",
+		}
+		restMapper := c.RESTMapper().(*meta.DefaultRESTMapper)
+		restMapper.Add(gvk, meta.RESTScopeNamespace)
+		return widget.Reconciler(c)
+	})
+```
+
+### Example: Using an explicit mapping
+
+[DefaultRESTMapper.AddSpecific](https://pkg.go.dev/k8s.io/apimachinery/pkg/api/meta@v0.28.4#DefaultRESTMapper.AddSpecific) 
+adds the GVK with an explicit resource name.
+
+```
+	rts.Run(t, scheme, func(t *testing.T, rtc *rtesting.ReconcilerTestCase, c reconcilers.Config) reconcile.Reconciler {
+		gvk := schema.GroupVersionKind{
+			Group:   "example.com",
+			Version: "v1alpha1",
+			Kind:    "Widget",
+		}
+		gvrSingular := schema.GroupVersionKind{
+			Group:    "example.com",
+			Version:  "v1alpha1",
+			Resource: "widget",
+		}
+     	gvrPlural := schema.GroupVersionKind{
+			Group:    "example.com",
+			Version:  "v1alpha1",
+			Resource: "widgets",
+		}  
+  
+		restMapper := c.RESTMapper().(*meta.DefaultRESTMapper)
+		restMapper.AddSpecific(gvk, gvrSingular, gvrPlural, meta.RESTScopeNamespace)
+		return widget.Reconciler(c)
+	})
+```