replication: update readme

replication/README.md

@@ -1,5 +1,4 @@
-osm/replication [![Godoc Reference](https://godoc.org/github.com/paulmach/osm/replication?status.svg)](https://godoc.org/github.com/paulmach/osm/replication)
-===============
+# osm/replication [![Godoc Reference](https://pkg.go.dev/badge/github.com/paulmach/osm)](https://pkg.go.dev/github.com/paulmach/osm/replication)
 
 Package `replication` handles fetching the Minute, Hour, Day and Changeset replication
 and the associated state value from [Planet OSM](http://planet.osm.org).
@@ -18,3 +17,15 @@ Once you know the change number you want, fetch the change using:
 ```go
 change, err := replication.Minute(ctx, num)
 ```
+
+## Finding sequences numbers by timestamp
+
+It's also possible to find the sequence number by timestamp.
+These calls make multiple requests for state to find the one matching the given timestamp.
+
+```go
+MinuteStateAt(ctx context.Context, timestamp time.Time) (MinuteSeqNum, *State, error)
+HourStateAt(ctx context.Context, timestamp time.Time) (HourSeqNum, *State, error)
+DayStateAt(ctx context.Context, timestamp time.Time) (DaySeqNum, *State, error)
+ChangesetStateAt(ctx context.Context, timestamp time.Time) (ChangesetSeqNum, *State, error)
+```