Add commitish to inputs in README

Author: IAmHughes

README.md

@@ -16,6 +16,7 @@ For more information on these inputs, see the [API Documentation](https://develo
 - `body_path`: A file with contents describing the release. Optional, and not needed if using `body`.
 - `draft`: `true` to create a draft (unpublished) release, `false` to create a published one. Default: `false`
 - `prerelease`: `true` to identify the release as a prerelease. `false` to identify the release as a full release. Default: `false`
+- `commitish` : Any branch or commit SHA the Git tag is created from, unused if the Git tag already exists. Default: SHA of current commit
 - `owner`: The name of the owner of the repo. Used to identify the owner of the repository.  Used when cutting releases for external repositories.  Default: Current owner
 - `repo`: The name of the repository. Used to identify the repository on which to release.  Used when cutting releases for external repositories. Default: Current repository