New introduction

Following an offline discussion with @aaronpk, I have added an intro to provide clarity that the levels are expected to be adhered to by RPs/apps.  It is likely this language needs further adjustment, readers should consider this as a starting place for further discussion.

ipsie-levels.md

@@ -1,4 +1,6 @@
-# IPSIE Levels
+# IPSIE 
+
+The capabilities documented in the levels below are specified as functionality for the relying party / applications to implement. In practice, enterprises that purchase SaaS applications should use these levels in RFPs to define the capabilities required of their applications.  The enterprise's identity systems (IdP, directory, etc.) may or may not support all of the functionality supported by a particular relying party / application.
 
 ## Session Lifecycle Management