Mention the situation when adding cloudron users to projects which have not yet logged in

Part of #2

POSTINSTALL.md

@@ -4,7 +4,8 @@ Taiga does not come with any global admin panel, every user creating a new proje
 
 === sso ===
 
-This app integrates with Cloudron authentication. Cloudron users can login and use Taiga.
+This app integrates with Cloudron authentication, however **before** adding Cloudron users to projects in Taiga,
+they have to login into the app once, otherwise the app assumes this is an external user and will send an invite.
 
 Taiga does not come with any global admin panel, every user creating a new project is the admin of that project.