Add specs of snippets from docs

spec/docs/mocks/overview_spec.cr

@@ -0,0 +1,40 @@
+require "../../spec_helper"
+
+Spectator.describe "Doubles" do
+  double :my_double do
+    stub answer { 42 }
+  end
+
+  specify "the answer to everything" do
+    dbl = double(:my_double)
+    expect(dbl.answer).to eq(42)
+  end
+end
+
+class MyType
+  def answer
+    123
+  end
+end
+
+Spectator.describe "Mocks" do
+  mock MyType do
+    stub answer { 42 }
+  end
+
+  specify "the answer to everything" do
+    m = MyType.new
+    expect(m.answer).to eq(42)
+  end
+end
+Spectator.describe "Mocks and doubles" do
+  double :my_double do
+    stub answer : Int32 # Return type required, otherwise nil is assumed.
+  end
+
+  specify "the answer to everything" do
+    dbl = double(:my_double)
+    allow(dbl).to receive(:answer).and_return(42)
+    expect(dbl.answer).to eq(42)
+  end
+end