Update README.md (#821)

* Update README.md

* Update README.md

Co-authored-by: Tatyana Kostromskaya <32135588+takost@users.noreply.github.com>

Co-authored-by: Tatyana Kostromskaya <32135588+takost@users.noreply.github.com>

README.md

@@ -82,6 +82,7 @@ Every argument is optional.
 | [ignore-updates](#ignore-updates)                                   | Any update (update/comment) can reset the stale idle time on the issues/PRs | `false`               |
 | [ignore-issue-updates](#ignore-issue-updates)                       | Override [ignore-updates](#ignore-updates) for issues only                  |                       |
 | [ignore-pr-updates](#ignore-pr-updates)                             | Override [ignore-updates](#ignore-updates) for PRs only                     |                       |
+| [include-only-assigned](#include-only-assigned)                             |    Process only assigned issues                |        `false`               |
 
 ### List of output options
 
@@ -517,6 +518,12 @@ Useful to override [ignore-updates](#ignore-updates) but only to ignore the upda
 
 Default value: unset
 
+#### include-only-assigned
+
+If set to `true`, only the issues or the pull requests with an assignee will be marked as stale automatically.
+
+Default value: `false`
+
 ### Usage
 
 See also [action.yml](./action.yml) for a comprehensive list of all the options.