add doc

README.md

@@ -67,6 +67,7 @@ jobs:
 - **new_version** - The value of the newly created tag without the prefix. Note that if there hasn't been any new commit, this will be `undefined`.
 - **previous_tag** - The value of the previous tag (or `v0.0.0` if none). Note that if `custom_tag` is set, this will be `undefined`.
 - **previous_version** - The value of the previous tag (or `0.0.0` if none) without the prefix. Note that if `custom_tag` is set, this will be `undefined`.
+- **release_type** - The computed release type (`major`, `minor`, `patch` or `custom` - can be prefixed with `pre`).
 - **changelog** - The [conventional changelog](https://github.com/conventional-changelog/conventional-changelog) since the previous tag.
 
 > **_Note:_** This action creates a [lightweight tag](https://developer.github.com/v3/git/refs/#create-a-reference) by default.