add CMDTEST file verifying doc example

doc/CMDTEST_example.rb

@@ -0,0 +1,20 @@
+class CMDTEST_example < Cmdtest::Testcase
+
+  def test_hello_world
+    cmd "echo hello" do
+      stdout_equal "hello\n"
+    end
+
+    cmd "echo WORLD" do
+      stdout_equal "world\n"
+    end
+  end
+
+  def test_touch_and_exit
+    cmd "touch bar.txt ; exit 8" do
+      created_files "foo.txt"
+      exit_status 7
+    end
+  end
+
+end