Add documentation for SHOW_REGISTRATION_BUTTON (#5778)

* Fix #5737

custom/conf/app.ini.sample

@@ -366,6 +366,8 @@ DEFAULT_ALLOW_ONLY_CONTRIBUTORS_TO_TRACK_TIME = true
 ; if he has set KeepEmailPrivate to true. The user's email will be replaced with a
 ; concatenation of the user name in lower case, "@" and NO_REPLY_ADDRESS.
 NO_REPLY_ADDRESS = noreply.example.org
+; Show Registration button
+SHOW_REGISTRATION_BUTTON = true
 
 [webhook]
 ; Hook task queue length, increase if webhook shooting starts hanging