Readme changes for example code and fixed header decoding

examples/simple_ex1.cc

@@ -0,0 +1,21 @@
+#include <iostream>
+#include "jwt/jwt.hpp"
+
+int main() {
+  using namespace jwt::params;
+
+  auto key = "secret"; //Secret to use for the algorithm
+  //Create JWT object
+  jwt::jwt_object obj{algorithm("HS256"), payload({{"some", "payload"}}), secret(key)};
+
+  //Get the encoded string/assertion
+  auto enc_str = obj.signature();
+  std::cout << enc_str << std::endl;
+
+  //Decode
+  auto dec_obj = jwt::decode(enc_str, algorithms({"HS256"}), secret(key));
+  std::cout << dec_obj.header() << std::endl;
+  std::cout << dec_obj.payload() << std::endl;
+
+  return 0;
+}