add docs about checking out the triggering ref (#21)

* add docs about checking out the triggering ref

* clarify checkout behavior

README.md

@@ -10,6 +10,7 @@ See [here](https://help.github.com/en/articles/events-that-trigger-workflows) to
 See [action.yml](action.yml)
 
 Basic:
+
 ```yaml
 steps:
 - uses: actions/checkout@master
@@ -20,6 +21,16 @@ steps:
 - run: npm test
 ```
 
+By default, the branch or tag ref that triggered the workflow will be checked out. If you wish to check out a different branch, specify that using `with.ref`:
+
+```yaml
+- uses: actions/checkout@master
+  with:
+    ref: some-branch
+```
+
+For more details, see [Contexts and expression syntax for GitHub Actions](https://help.github.com/en/articles/contexts-and-expression-syntax-for-github-actions)
+
 # License
 
 The scripts and documentation in this project are released under the [MIT License](LICENSE)