Update mailroom README description

Use the landing page copy to better describe the two
server-side roles bundled by payjoin-mailroom, and drop
the now-broken links to payjoin-directory and ohttp-relay.

Author: spacebear21

payjoin-mailroom/README.md

@@ -1,6 +1,9 @@
 # Payjoin Mailroom
 
-The Payjoin Mailroom is a unified Payjoin Directory and OHTTP Relay server. Combines [payjoin-directory](../payjoin-directory/README.md) and [ohttp-relay](../ohttp-relay/README.md) into a single binary.
+payjoin-mailroom is a single, lightweight binary that bundles the two server-side roles required by BIP 77 Async Payjoin:
+
+- **Payjoin Directory**: a store-and-forward mailbox that holds small, ephemeral, end-to-end encrypted payloads so a sender and receiver can complete a payjoin asynchronously (they don't need to be online at the same time).
+- **OHTTP Relay**: an [Oblivious HTTP](https://en.wikipedia.org/wiki/Oblivious_HTTP) proxy that separates client IP addresses from the directory, preventing the directory from correlating users with their network identity.
 
 Note that this binary is under active development and thus the CLI and configuration file may be unstable.