[skip ci] Early mention of the async accesses

Author: groue

README.md

@@ -491,6 +491,8 @@ let newPlaceCount = try dbQueue.write { db -> Int in
 }
 ```
 
+See the [Concurrency] guide for asynchronous database accesses.
+
 **A database queue serializes accesses to the database**, which means that there is never more than one thread that uses the database.
 
 - When you don't need to modify the database, prefer the `read` method. It prevents any modification to the database.
@@ -548,6 +550,8 @@ let newPlaceCount = try dbPool.write { db -> Int in
 }
 ```
 
+See the [Concurrency] guide for asynchronous database accesses.
+
 **Database pools allow several threads to access the database at the same time:**
 
 - When you don't need to modify the database, prefer the `read` method, because several threads can perform reads in parallel.