update the readme so it's clearer for current state of relay

README.md

@@ -1,19 +1,22 @@
 # cerulea-relay
 
-Realtime relay (1hr backfill window) for PDSes with fewer than 1000 repos.
+Realtime non-archival relay for third-party AT Proto PDSes.
 
-The idea is that we can have much larger limits if we scale down the volume of the network.
+In the interest of cost control, we are scaling down the network:
+- Only PDSes with fewer than 1000 repos are crawled
+- We do no backfilling, only current events are relayed to consumers
+- Stale data (≈ 24hrs?) is purged from the database [not doing this yet]
+
+The idea is that we can have apps with much larger limits if we scale down the volume of the network.
 - Large block sizes
 - Large record size limit
+- therefore: Large text field in post records, large uploads
 - etcetcetc
 
 ## todo
 
 - metrics / tracing / observability shit
-- keep track of currently subscribed PDSes in application state
-- discover PDSes via crawling instead of hardcoding 1 (lol)
-    - do not allow PDSes with more than 1000 repos
 - history:
-    - timestamp instead of seq number as key ?
-    - purge based on ttl
-    - takedowns probably
+  - store indexedAt values
+  - purge based on ttl
+  - takedowns