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 applies
the changes to its own tables and answers from them. The section says
those tables hold only the routes announced since routewatch first
started, and miss the updates it drops under load or while it is
stopped.

Model: opus-5-5

README.md

@@ -5,6 +5,22 @@ table in real time: it streams BGP UPDATE messages from the RIPE RIS Live
 service, maintains a live routing table in SQLite, and provides HTTP APIs for
 querying routing information.
 
+## Rationale
+
+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+route the feed has withdrawn.
+
 ## Features
 
 - Real-time streaming of BGP updates from RIPE RIS Live