add tip for registering a GitHub OAuth application

options/locale/locale_en-US.ini

@@ -1089,6 +1089,7 @@ auths.oauth2_clientID = Client ID (Key)
 auths.oauth2_clientSecret = Client Secret
 auths.enable_auto_register = Enable Auto Registration
 auths.tips = Tips
+auths.tip.github = Register a new OAuth application on https://github.com/settings/applications/new and use <host>/user/oauth2/github/callback as "Authorization callback URL"
 auths.edit = Edit Authentication Setting
 auths.activated = This authentication is activated
 auths.new_success = New authentication '%s' has been added successfully.

templates/admin/auth/new.tmpl

@@ -195,6 +195,8 @@
 				<div class="ui attached segment">
 					<h5>GMail Settings:</h5>
 					<p>Host: smtp.gmail.com, Port: 587, Enable TLS Encryption: true</p>
+					<h5>OAuth GitHub:</h5>
+					<p>{{.i18n.Tr "admin.auths.tip.github"}}</p>
 				</div>
 			</div>
 		</div>