Document SAL_RTL_ENABLED env var

Change-Id: I6242f6028daaa0eb4d14134658e2eb50fd8f8275
Reviewed-on: https://gerrit.libreoffice.org/45932Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>
Tested-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

vcl/README.vars

@@ -3,6 +3,7 @@ Environment variables in VCL:
 General
 -------
 SAL_USE_VCLPLUGIN - use a VCL plugin
+SAL_RTL_ENABLED - Enable RTL UI
 SAL_NO_NWF - disable native widgets
 SAL_FORCEDPI - force a specific DPI (gtk & gtk3 plugins only)
 SAL_FORCE_HC - force high-contrast mode