README: document read/write lock concurrency model

README.md

@@ -46,9 +46,12 @@ mongosh --port 27017
   Killed mid-write (`kill -9`), the database recovers all committed writes;
   the log and compaction both work with relative or absolute `--db` paths.
   Records up to the announced 16 MB `maxBsonObjectSize` replay correctly.
-- **Concurrency**: one mutex serializes commands end-to-end (command-level
-  atomicity); the Io worker pool only overlaps connection I/O with command
-  execution — there is no read parallelism. Fine for light workloads.
+- **Concurrency**: a writer-preferring read/write lock splits command
+  execution — reads (`find`, `count`, `aggregate`, `list*`) run concurrently
+  across connections, writes (CRUD, DDL) are exclusive and totally ordered,
+  and handshake/no-op commands run lock-free. The log append + `fsync` still
+  happen under the write lock, so the crash guarantees are unchanged. Fine
+  for light workloads.
 
 ## Layout