chore(docs): example of unsafe header decode in README (#501)

Author: saranshdhingra

Diff for: README.md

@@ -65,6 +65,40 @@ $decoded_array = (array) $decoded;
 JWT::$leeway = 60; // $leeway in seconds
 $decoded = JWT::decode($jwt, new Key($key, 'HS256'));
 ```
+Example encode/decode headers
+-------
+Decoding the JWT headers without verifying the JWT first is NOT recommended, and is not supported by
+this library. This is because without verifying the JWT, the header values could have been tampered with.
+Any value pulled from an unverified header should be treated as if it could be any string sent in from an
+attacker.  If this is something you still want to do in your application for whatever reason, it's possible to 
+decode the header values manually simply by calling `json_decode` and `base64_decode` on the JWT 
+header part:
+```php
+use Firebase\JWT\JWT;
+
+$key = 'example_key';
+$payload = [
+    'iss' => 'http://example.org',
+    'aud' => 'http://example.com',
+    'iat' => 1356999524,
+    'nbf' => 1357000000
+];
+
+$headers = [
+    'x-forwarded-for' => 'www.google.com'
+];
+
+// Encode headers in the JWT string
+$jwt = JWT::encode($payload, $key, 'HS256', null, $headers);
+
+// Decode headers from the JWT string WITHOUT validation
+// **IMPORTANT**: This operation is vulnerable to attacks, as the JWT has not yet been verified.
+// These headers could be any value sent by an attacker.
+list($headersB64, $payloadB64, $sig) = explode('.', $jwt);
+$decoded = json_decode(base64_decode($headersB64), true);
+
+print_r($decoded);
+```
 Example with RS256 (openssl)
 ----------------------------
 ```php