Readme: Added explanation of sign on options

Author: Jefferson49

README.md

@@ -11,6 +11,8 @@ This README file contains the following main sections:
 +   [IMPORTANT SECURITY NOTES](#important-security-notes)
 +   [Installation](#installation)
 +   [Supported Authorization Providers](#supported-authorization-providers)
++   [Sign On Options](#sign-on-options)
++   [Handling of User Name and Email](#handling-of-user-name-and-email)
 +   [Configuration of Authorization Providers](#configuration-of-authorization-providers)
     + [General Configuration](#general-configuration)
     + [Generic](#generic)
@@ -57,6 +59,16 @@ Please check whether your **webtrees BASE_URL** in the config.ini.php file **sta
 	+ Go to "Control Panel/All Modules", and find the module called "ExtendedImportExport"
 	+ Check if it has a tick for "Enabled"
 
+## Sign On Options
+
+The OAuth 2.0 Client offers two different options to sign on with an authorization provider:
++ **Register a new webtrees user based on the identity provided by an authorization provider**: Sign out and choose to sign in with an authorization provider. A new webtrees user will be registered. The registration will fail if the email oder the user name, which is transmitted by the authorization provider, already exists within webtrees. After the registration, a webtrees administrator needs to approve the new user and the email before signing in with the authorization provider is possible. The general idea of this option is that the user identify from the authorization provider is used within webtrees. In the control panel, a setting is offered to synchronize the email address from the authorization provider to the webtrees user.
++ **Connect an existing webtrees user with an authorization provider**: Sign on with an exisiting user and choose to "connect" the existing user with an authorization provider. In this case, the existing user credentials in webtrees will continue to exist and the user will be (additionally) allowed to sign in with the chosen provider. The general idea of this is that the two user accounts exist in parallel and are connected.
+
+## Handling of User Name and Email
+
+If a new webtrees user is created (i.e. registered), the user name and the email needs to be unique. This is a general webtrees requirement. If the user name or email address provided by the authorization provider is identical to an already existing user, an error message will be shown. 
+
 ## Supported Authorization Providers
 The OAuth 2.0 Client for webtrees uses the OAuth 2.0 implementation of the [The League](https://oauth2-client.thephpleague.com/), which allows developers to create OAuth 2.0 clients that interface with a wide-variety of OAuth 2.0 providers. Within this concept, The League supports serveral "[official providers](https://oauth2-client.thephpleague.com/providers/league/)". As far as suitable for webtrees, these official providers were included, see list below.