Add input documentation

Author: m-kuhn

README.md

@@ -15,3 +15,16 @@ This action is a fork of https://github.com/tibdex/backport, it adds support for
 
 _Note:_ multiple backport labels can be added.
 For example, if a pull request has the labels `backport staging` and `backport production` it will be backported to both branches: `staging` and `production`.
+
+
+## Customizing
+
+### inputs
+
+Following inputs can be used as `step.with` keys
+
+| Name               | Type    | Description                       |
+|--------------------|---------|-----------------------------------|
+| `add_labels`       | String  | Comma separated list of labels to add to the backport PR. |
+| `github_token`     | String  | Token for the GitHub API. Set this to a personal access token (PAT) if you want github workflows to run on the opened PR. |
+| `title_template`   | String  | Template for the title of the opened PR. E.g. `[Backport {{base}}] {{originalTitle}}` |