Clarify version number for which the example works

adnanh#461 changed option name and in adnanh#528 (comment), @moorereason suggests to look at old tags of example documentation. This would mean that users have to read through random old documentation to discover why their packaged version doesn't work . Suggesting that clarity in the examples is preferable.

Recall that renaming this doesn't give the user some easy exception. It just mean that the trigger isn't satisfied, so there are A LOT of options for debugging.

(which takes a lot of time to do, so that's why this information is important)

docs/Hook-Examples.md

@@ -23,6 +23,9 @@ although the examples on this page all use the JSON format.
 * [Pass string arguments to command](#pass-string-arguments-to-command)
 
 ## Incoming Github webhook
+
+This example works on 2.8+ versions of Webhook - if you are on a previous series, change `payload-hmac-sha1` to `payload-hash-sha1`.
+
 ```json
 [
   {