fix: Add link to Slack's Advanced Formatting Documentation for messag…

…es (#593)

docs/user-guide/metadata.md

@@ -250,6 +250,8 @@ Result:
 
 You can customize [notification](./configuration/settings.html#slack) messages with meta. Meta keys are different for each notification plugin.
 
+You will need special string formatting for mentions or channel links in Slack. You can read more about the available options in the [Slack Documentation](https://api.slack.com/reference/surfaces/formatting#advanced).
+
 #### Basic
 Example screwdriver.yaml notifying with Slack:
 ```yaml
@@ -424,4 +426,4 @@ meta.set(key, array)
 -- print the index
 -- NOTE: index for meta.(get/set) purposes is 0-based, so subtract 1 from the array size
 print(#array - 1)
-```
+```