Update quickstart demo

Signed-off-by: Clifford Wolf <clifford@clifford.at>

docs/examples/quickstart/demo.sv

@@ -0,0 +1,19 @@
+module demo (
+  input clk,
+  output [5:0] counter
+);
+  reg [5:0] counter = 0;
+
+  always @(posedge clk) begin
+    if (counter == 50)
+      counter <= 0;
+    else
+      counter <= counter + 1;
+  end
+
+`ifdef FORMAL
+  always @(posedge clk) begin
+    assert (counter < 32);
+  end
+`endif
+endmodule