save docs for adding your own algo

docs/signing.md

@@ -0,0 +1,34 @@
+# Signing Tokens
+
+## Custom Signature Algorithms
+
+The libraries design is open so you can implement your own algorithms, see [existing examples](https://github.com/Thalhammer/jwt-cpp/blob/73f23419235661e89a304ba5ab09d6714fb8dd94/include/jwt-cpp/jwt.h#L874) for ideas.
+
+```cpp
+struct your_algorithm{
+	std::string sign(const std::string& /*unused*/, std::error_code& ec) const {
+		ec.clear();
+        // CALL YOUR METHOD HERE
+		return {};
+	}
+	void verify(const std::string& /*unused*/, const std::string& signature, std::error_code& ec) const {
+		ec.clear();
+		if (!signature.empty()) { ec = error::signature_verification_error::invalid_signature; }
+        
+        // CALL YOUR METHOD HERE
+	}
+	std::string name() const { return "your_algorithm"; }
+};
+```
+
+Then everything else is the same, just pass in your implementation such as:
+
+
+```cpp
+auto token = jwt::create()
+                .set_id("custom-algo-example")
+                .set_issued_at(std::chrono::system_clock::now())
+                .set_expires_at(std::chrono::system_clock::now() + std::chrono::seconds{36000})
+                .set_payload_claim("sample", jwt::claim(std::string{"test"}))
+                .sign(your_algorithm{/* what ever you want */});
+```