chore: add call back validation javadoc and readme code example

README.md

@@ -229,6 +229,21 @@ Current status for submission 43cf7163-5163-4bc8-865e-be96e271ecc3 => incomplete
 
 <img src="https://docs.fitko.de/fit-connect/assets/images/status-ebe91122f32321e22f094882a66c1139.svg">
 
+### Validating callbacks
+
+Usage 
+
+```java
+final SenderClient senderClient = ClientFactory.senderClient(config);
+
+final ValidationResult validationResult = senderClient.validateCallback("hmac", 0L, "body", "secret");
+
+if(validationResult.hasError()){
+    LOGGER.error(validationresult.getError().getMessage());
+}
+```
+
+
 ## API Usage for Subscriber
 
 ### Retrieving submissions

client/src/main/java/dev/fitko/fitconnect/client/SenderClient.java

@@ -70,8 +70,17 @@ public class SenderClient {
         return sender.getLastedEvent(destinationId, caseId, submissionId, authenticationTags);
     }
 
-    public ValidationResult validateCallback(String hmac, Long timestamp, String httpBody, String callbackSecret) {
-        return this.sender.validateCallback(hmac, timestamp, httpBody, callbackSecret);
+    /**
+     * Checks if a received callback can be trusted by validating the provided request data.
+     *
+     * @param hmac           authentication code provided by the callback
+     * @param timestamp      timestamp provided by the callback
+     * @param httpBody       HTTP body provided by the callback
+     * @param callbackSecret secret owned by the client, which is used to calculate the hmac
+     * @return {@code true} if hmac and timestamp provided by the callback meet the required conditions
+     */
+    public ValidationResult validateCallback(final String hmac, final Long timestamp, final String httpBody, final String callbackSecret) {
+        return sender.validateCallback(hmac, timestamp, httpBody, callbackSecret);
     }
 
     /**