Add note in README

Author: gcatanese

tools/hmac/README.md

@@ -3,6 +3,8 @@
 This folder contains PHP scripts to calculate the HMAC signature of the webhook payload.  
 They can be used to troubleshoot the HMAC signature calculation.
 
+Note: make sure you are using the HMAC key used to generate the signature associated with the payload in the JSON file
+
 ### Payments webhooks
 
 Copy the content of the webhook in the payload.json (or provide a different file), then run with: