Say in README why RouteWatch exists (closes #204)

The Rationale section now gives the owner's reason for the program: it
is an internal API that our other services ask about IP addresses. It
sees which AS routes each address and which announced prefix holds it
from its own routing table. WHOIS details of each AS, kept in its
database, add to that.

What RIS Live does and does not give stays, shortened. Planned geojs
lookups are left out, because the README describes only what is built.

Model: opus-5-5

TODO.md

@@ -35,6 +35,10 @@ tested.
 
 # Completed Steps
 
+- 2026-10-27: README "Rationale" says why RouteWatch exists: to answer our other
+  services' questions about IP addresses, from its own routing table and the
+  WHOIS details it keeps. It still says what RIS Live does not give (closes
+  #204)
 - 2026-10-27: every JSON answer carries `@meta`. A panic on a JSON route answers
   500 with the JSON error body. `/status.json` and the health check answer 405
   with the JSON error body to any method they do not take. The 500 and the 405