Standalone envtest example

This adds a standalone example of envtest not being used in testing.
It's super useful for interactive testing of controller and webhook
behavior, and code I've written in various forms for a one-off at least
a dozen times.

examples/scratch-env/go.mod

@@ -0,0 +1,11 @@
+module sigs.k8s.io/controller-runtime/examples/scratch-env
+
+go 1.15
+
+require (
+	github.com/spf13/pflag v1.0.5
+	k8s.io/client-go v0.19.2
+	sigs.k8s.io/controller-runtime v0.0.0-00010101000000-000000000000
+)
+
+replace sigs.k8s.io/controller-runtime => ../..