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

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -206,6 +206,7 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `ENABLE_USER_HEATMAP`: **true** Enable this to display the heatmap on users profiles.
 - `EMAIL_DOMAIN_WHITELIST`: **\<empty\>**: If non-empty, list of domain names that can only be used to register
   on this instance.
+- `SHOW_REGISTRATION_BUTTON`: **! DISABLE\_REGISTRATION**: Show Registration Button
 
 ## Webhook (`webhook`)