doc: add SAML IdP deprecation notice

* Also small edits to the deprecation notice that's already there.

README.md

@@ -2,7 +2,9 @@
 
 **Slack** [#uaa](https://cloudfoundry.slack.com/archives/C03FXANBV)
 
-> **_Warning_**: _MFA feature in UAA is currently deprecated and will be removed in a future UAA version._
+> **_Deprecation notice_**: _UAA can currently function as a SAML identity provider (IdP). This functionality will be removed in a future release. This will facilitate ongoing efforts to upgrade the Spring Boot library that UAA uses. However, UAA's ability to be a service provider (SP) that integrates with an external SAML-based IdP will still be supported._
+ 
+> **_Deprecation notice_**: _The MFA feature in UAA is deprecated and will be removed in a future UAA version._
 
 # CloudFoundry User Account and Authentication (UAA) Server