docs: add note about skip CI semantics [skip ci]

closes #16

Signed-off-by: Rui Chen <rui@chenrui.dev>

README.md

@@ -4,3 +4,9 @@
 ![repository-dispatch](https://github.com/chenrui333/github-action-test/workflows/repository-dispatch/badge.svg)
 steps-condition-test
 [![docker](https://github.com/chenrui333/github-action-test/actions/workflows/docker.yml/badge.svg)](https://github.com/chenrui333/github-action-test/actions/workflows/docker.yml)
+
+## skip CI semantics
+
+I used to do something custom like [this](https://github.com/chenrui333/github-action-test/pull/16), but [now it is native supproted][skip-ci-semantics].
+
+[skip-ci-semantics]: https://github.blog/changelog/2021-02-08-github-actions-skip-pull-request-and-push-workflows-with-skip-ci/