Show new mock and double system in README

spec/docs/readme_spec.cr

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+require "../spec_helper"
+
+private abstract class Interface
+  abstract def invoke(thing) : String
+end
+
+# Type being tested.
+private class Driver
+  def do_something(interface : Interface, thing)
+    interface.invoke(thing)
+  end
+end
+
+Spectator.describe Driver do
+  # Define a mock for Interface.
+  mock Interface
+
+  # Define a double that the interface will use.
+  double(:my_double, foo: 42)
+
+  it "does a thing" do
+    # Create an instance of the mock interface.
+    interface = mock(Interface)
+    # Indicate that `#invoke` should return "test" when called.
+    allow(interface).to receive(:invoke).and_return("test")
+
+    # Create an instance of the double.
+    dbl = double(:my_double)
+    # Call the mock method.
+    subject.do_something(interface, dbl)
+    # Verify everything went okay.
+    expect(interface).to have_received(:invoke).with(dbl)
+  end
+end