write_cxxrtl: add basic documentation.

Author: whitequark

backends/cxxrtl/cxxrtl.cc

@@ -1601,7 +1601,22 @@ struct CxxrtlBackend : public Backend {
 		log("\n");
 		log("    write_cxxrtl [options] [filename]\n");
 		log("\n");
-		log("Write C++ code for simulating the design.\n");
+		log("Write C++ code for simulating the design. The generated code requires a driver;\n");
+		log("the following simple driver is provided as an example:\n");
+		log("\n");
+		log("    #include \"top.cc\"\n");
+		log("\n");
+		log("    int main() {\n");
+		log("      cxxrtl_design::p_top top;\n");
+		log("      while (1) {\n");
+		log("        top.p_clk.next = value<1> {1u};\n");
+		log("        top.step();\n");
+		log("        top.p_clk.next = value<1> {0u};\n");
+		log("        top.step();\n");
+		log("      }\n");
+		log("    }\n");
+		log("\n");
+		log("The following options are supported by this backend:\n");
 		log("\n");
 		log("    -O <level>\n");
 		log("        set the optimization level. the default is -O%d. higher optimization\n", DEFAULT_OPT_LEVEL);