GitHub Apps OAuth Web Flow: document allow_signup parameter

Author: gr2m

content/developers/apps/identifying-and-authorizing-users-for-github-apps.md

@@ -50,6 +50,7 @@ Name | Type | Description
 `redirect_uri` | `string` | The URL in your application where users will be sent after authorization. This must be an exact match to {% if currentVersion == "free-pro-team@latest" or currentVersion ver_gt "enterprise-server@3.0" %} one of the URLs you provided as a **Callback URL** {% else %} the URL you provided in the **User authorization callback URL** field{% endif %} when setting up your GitHub App and can't contain any additional parameters.
 `state` | `string` | This should contain a random string to protect against forgery attacks and could contain any other arbitrary data.
 `login` | `string` | Suggests a specific account to use for signing in and authorizing the app.
+`allow_signup` | `string` | Whether or not unauthenticated users will be offered an option to sign up for GitHub during the OAuth flow. The default is `true`. Use `false` when a policy prohibits signups.
 
 {% note %}