Update private preview TOS

Author: Caitlin Bales (MSFT)

README.md

@@ -1,5 +1,11 @@
 # Preview Microsoft Graph SDK for Java
-*This client library is currently in preview status. We welcome your feedback as we drive the quality of this to general availability.*
+This client library is currently in private preview status. This means that: 
+- The API surface may change significantly
+- Non-blocking issues *may* be triaged at a slower pace
+- You may submit feedback through issues or PRs. You will have a better chance of your changes being implemented if they are submitted before public preview release
+- We do not define an SLA or support strategy for this library
+
+We welcome your feedback as we drive the quality of this to general availability.
 
 # Get started with the Microsoft Graph SDK for Java