Add the Rationale section to README (closes #121)

REPO_POLICIES.md requires a Rationale section saying why the program
exists. RIS Live sends only the changes to the routing table, so it
cannot answer which AS an address is routed to now; routewatch keeps
the table the changes add up to and answers from it.

Model: opus-5-5

README.md

@@ -10,16 +10,10 @@ querying routing information.
 RIPE RIS Live streams the BGP updates that RIPE's route collectors receive, as
 they happen, from the moment a client connects. It reports changes, not the
 routing table they add up to, so it cannot say which AS an IP address is routed
-to now or which routes a prefix has. RouteWatch applies the feed's updates to
-live routing tables in its own SQLite database and answers IP-to-AS lookups and
-prefix pages from them.
-
-The feed sends no table when a client connects, so these tables hold only the
-routes the feed has announced since RouteWatch first started: an IP-to-AS lookup
-finds nothing for an address whose routes have not been announced since then.
-The tables also miss the updates RouteWatch drops when it falls behind under
-load and those sent while it is stopped, so a lookup or a prefix page can show a
-route the feed has withdrawn.
+to now or which routes a prefix has. RouteWatch keeps that table: it applies the
+feed's updates to live routing tables in its own SQLite database and answers
+from them, so an IP-to-AS lookup or a prefix page shows the routes as the feed
+last reported them.
 
 ## Features