Added second example for readme

examples/simple_ex2.cc

@@ -0,0 +1,50 @@
+#include <chrono>
+#include <cassert>
+#include <iostream>
+#include "jwt/jwt.hpp"
+
+int main() {
+  using namespace jwt::params;
+
+  jwt::jwt_object obj{algorithm("hs256"), secret("secret"), payload({{"user", "admin"}})};
+
+  //Use add_claim API to add claim values which are
+  // _not_ strings.
+  // For eg: `iat` and `exp` claims below.
+  // Other claims could have been added in the payload
+  // function above as they are just stringy things.
+  obj.add_claim("iss", "arun.muralidharan")
+     .add_claim("sub", "test")
+     .add_claim("id", "a-b-c-d-e-f-1-2-3")
+     .add_claim("iat", 1513862371)
+     .add_claim("exp", std::chrono::system_clock::now() + std::chrono::seconds{10})
+     ;
+
+  //Use `has_claim` to check if the claim exists or not
+  assert (obj.has_claim("iss"));
+  assert (obj.has_claim("exp"));
+
+  //Use `has_claim_with_value` to check if the claim exists
+  //with a specific value or not.
+  assert (obj.payload().has_claim_with_value("id", "a-b-c-d-e-f-1-2-3"));
+  assert (obj.payload().has_claim_with_value("iat", 1513862371));
+
+  //Remove a claim using `remove_claim` API.
+  //Most APIs have an overload which takes enum class type as well
+  //It can be used interchangeably with strings.
+  obj.remove_claim(jwt::registered_claims::expiration);
+  assert (not obj.has_claim("exp"));
+
+  //Using `add_claim` with extra features.
+  //Check return status and overwrite
+  bool ret = obj.payload().add_claim("sub", "new test", false/*overwrite*/);
+  assert (not ret);
+
+  // Overwrite an existing claim
+  ret = obj.payload().add_claim("sub", "new test", true/*overwrite*/);
+  assert ( ret );
+
+  assert (obj.payload().has_claim_with_value("sub", "new test"));
+
+  return 0;
+}