Add example command for collecting artifacts (#1671)

Co-authored-by: Markus Wolf <KnisterPeter@users.noreply.github.com>

README.md

@@ -164,6 +164,9 @@ act pull_request
 # Run a specific job:
 act -j test
 
+# Collect artifacts to the /tmp/artifacts folder:
+act --artifact-server-path /tmp/artifacts
+
 # Run a job in a specific workflow (useful if you have duplicate job names)
 act -j lint -W .github/workflows/checks.yml