feat: add extra-files input (#410)

README.md

@@ -55,6 +55,7 @@ Automate releases with Conventional Commit Messages.
 | `monorepo-tags` | add prefix to tags and branches, allowing multiple libraries to be released from the same repository. |
 | `changelog-types` | A JSON formatted String containing to override the outputted changelog sections |
 | `version-file` | provide a path to a version file to increment (used by ruby releaser) |
+| `extra-files` | add extra-files to bump using the [generic updater](https://github.com/googleapis/release-please/blob/main/docs/customizing.md#updating-arbitrary-files) |
 | `fork`          | Should the PR be created from a fork. Default `false`|
 | `command`          | release-please command to run, either `github-release`, or `release-pr`, `manifest`, `manifest-pr` (_defaults to running both_) |
 | `default-branch`  | branch to open pull release PR against (detected by default) |
@@ -271,6 +272,30 @@ jobs:
           git push origin v${{ steps.release.outputs.major }}.${{ steps.release.outputs.minor }}
 ```
 
+## Adding additional files
+
+You can update additional files with the `extra-files` input.
+
+See the [generic updater docs](https://github.com/googleapis/release-please/blob/main/docs/customizing.md#updating-arbitrary-files) for more info on how release-please will update those files.
+
+```yaml
+on:
+  push:
+    branches:
+      - main
+name: release-please
+jobs:
+  release-please:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: GoogleCloudPlatform/release-please-action@v3
+        with:
+          release-type: node
+          extra-files: |
+            README.md
+            docs/getting-started.md
+```
+
 ## License
 
 Apache Version 2.0