add CI_BRANCH in README
@@ -323,6 +323,7 @@ You can use reviewdog to post review comments anywhere with following environmen
| `CI_COMMIT` | SHA1 for the current build |
| `CI_REPO_OWNER` | repository owner (e.g. "haya14busa" for https://github.com/haya14busa/reviewdog) |
| `CI_REPO_NAME` | repository name (e.g. "reviewdog" for https://github.com/haya14busa/reviewdog) |
+| `CI_BRANCH` | [optional] branch of the commit |
| `REVIEWDOG_GITHUB_API_TOKEN` | GitHub Personal API Access token |
```sh
