docs: Add notes on issue assignment in CONTRIBUTING.md (argoproj#12678)

Signed-off-by: Yuan (Terry) Tang <[email protected]>

docs/CONTRIBUTING.md

@@ -123,6 +123,8 @@ Bugs can be [sorted by "👍"](https://github.com/argoproj/argo-workflows/issues
 If the issue is determined to be a user error and not a bug, remove the `bug` label (and the `regression` label, if applicable) and replace it with the `support` label.
 If more information is needed from the author to diagnose the issue, then apply the `more information needed` label.
 
+Please only assign issues to members. New contributors are encouraged to work on a PR directly without being assigned.
+
 ##### Staleness
 
 Only issues and PRs that have the [`more information needed` label](https://github.com/argoproj/argo-workflows/labels/more%20information%20needed) will be considered for staleness.