Add an example how to use it

[certik]

I created an example by copy & pasting things from the README:

#include "include/argparse.hpp"

int main(int argc, char *argv[]) {
	argparse::ArgumentParser program("test");

	program.add_argument("square")
	  .help("display the square of a given number")
	  .action([](const std::string& value) { return std::stoi(value); });

	program.add_argument("--verbose")
	  .default_value(false)
	  .implicit_value(true);

	program.parse_args(argc, argv);

	int input = program.get<int>("square");

	if (program["--verbose"] == true) {
	  std::cout << "The square of " << input << " is " << (input * input) << std::endl;
	}
	else {
	  std::cout << (input * input) << std::endl;
	}

	return 0;
}

And it seems to fail:

$ ./a.out 4
16
$ ./a.out --help
terminate called after throwing an instance of 'std::runtime_error'
  what():  help called
Aborted (core dumped)

[p-ranav]

Updated README for usage. Please test, confirm and close.

[certik]

I think it works now, thanks! I get:

$ ./a.out --help
help called
Usage: test [options] square 

Positional arguments:
square    	display the square of a given number

Optional arguments:
-h --help 	show this help message and exit
--verbose