Add a brief reference to the perspectives project and mention what should happen if a server goes offline

specification/30_server_server_api.rst

@@ -81,6 +81,13 @@ directly or by querying an intermediate perspective server using a
 response with their own key. A server may query multiple perspective servers
 to ensure that they all report the same public keys.
 
+This approach is borrowed from the Perspectives Project
+(http://perspectives-project.org/), but modified to include the NACL keys and to
+use JSON instead of XML. It has the advantage of avoiding a single trust-root
+since each server is free to pick which perspective servers they trust and can
+corroborate the keys returned by a given perspective server by querying other
+servers.
+
 Publishing Keys
 _______________
 
@@ -117,6 +124,10 @@ remaining life time to avoid serving a stale response. Servers should avoid
 querying for certificates more frequently than once an hour to avoid flooding
 a server with requests.
 
+If a server goes offline intermediate perspective servers should continue to
+return the last response they received from that server so that the signatures
+of old events sent by that server can still be checked.
+
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     Key                    Type                         Description
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