README: Update list issues concurrent read/writes

Update the README FAQ section adding one issues more to the list of the
issues related with the concurrent reads and writes. This new added
issues gives a lot of insights, much more than the ones which where
already present.

Author: ifraixedes

README.md

@@ -69,7 +69,7 @@ FAQ
 
 * Can I use this in multiple routines concurrently?
 
-    Yes for readonly. But, No for writable. See [#50](https://github.com/mattn/go-sqlite3/issues/50), [#51](https://github.com/mattn/go-sqlite3/issues/51), [#209](https://github.com/mattn/go-sqlite3/issues/209).
+    Yes for readonly. But, No for writable. See [#50](https://github.com/mattn/go-sqlite3/issues/50), [#51](https://github.com/mattn/go-sqlite3/issues/51), [#209](https://github.com/mattn/go-sqlite3/issues/209), [#274](https://github.com/mattn/go-sqlite3/issues/274).
 
 * Why is it racy if I use a `sql.Open("sqlite3", ":memory:")` database?