Update report-ci-errors README

.github/actions/report-ci-errors/README.md

@@ -4,7 +4,11 @@ You will need to get the slack webhook url from the secrets manager first and ad
 
 ## Usage example:
 
-Note: Typically you will want to report CI errors as part of a deployment workflow. The code below demonstrates where to insert the lines and what lines to copy across.
+Notes: 
+  - To minimise noise the slack message is only sent out on the main branch.
+  - Typically you will want to report CI errors as part of a deployment workflow. 
+
+The code below indicates where to insert the lines and what lines to copy across.
 
 ```
 name: Always failing job