Add warning for incompatible emails

Added an aside to the prerequisite section to explain that plus-addressed emails (which can be used for your Cloudflare account when signing in with a username and password) are incompatible with Dashboard SSO.

content/cloudflare-one/applications/configure-apps/dash-sso-apps.md

@@ -16,6 +16,12 @@ By adding a Dashboard SSO application to your Cloudflare Zero Trust account, you
 
 All users in your email domain must exist as a member in your Cloudflare account and IdP.  To add users to your Cloudflare account, refer to [Manage Cloudflare account access](/fundamentals/account-and-billing/members/).
 
+{{<Aside>}}
+
+Cloudflare dashboard SSO does not support users with plus-addressed emails, such as `[email protected]`. If you have users like this added to your Cloudflare organization, they will be unable to login with SSO.
+
+{{</Aside>}}
+
 ## 1. Set up an IdP
 
 Add an IdP to Cloudflare Zero Trust by following [our detailed instructions](/cloudflare-one/identity/idp-integration/).