docs: HTTPS_CALLBACK env var

Author: Clem-Fern

README.md

@@ -113,7 +113,8 @@ rtabby-web-api store tabby's configuration in a database. You can choose between
     |BIND_PORT|Port listening on (Optional)|8989|8080|
     |SSL_CERTIFICATE|Server certificate (Optional)|cert.pem|None|
     |SSL_CERTIFICATE_KEY|Server certificate private key(Optional)|private.key|None|    
-    |CLEANUP_USERS|Delete configurations own by unknown user (Be careful)(Optional)|true|false|  
+    |CLEANUP_USERS|Delete configurations own by unknown user (Be careful)(Optional)|true|false|
+    |HTTPS_CALLBACK|Third party login, enable https on callback uri(Optional)|true|false|  
 
 ## Usage