Add examples for signature

[Rossb0b]

Actually, there is only one example of how to deal with our generated signature and this one is in JS.

  const { headers, body } = req;

  const { signature, date } = headers;

  const generatedSignature = createHmac("sha256", kSecret)
    .update(JSON.stringify(body) + date)
    .digest("hex");

  if (signature !== generatedSignature) {
    throw new Error("Wrong signature");
  }

It would be great to have this particular code in those languages :

• C++
• Java
• Python