Add 'async-session-r2d2'

Author: jalcine

README.md

@@ -46,6 +46,7 @@
 * [async-sqlx-session](https://crates.io/crates/async-sqlx-session) postgres & sqlite
 * [async-redis-session](https://crates.io/crates/async-redis-session)
 * [async-mongodb-session](https://crates.io/crates/async-mongodb-session)
+* [async-session-r2d2](https://crates.io/crates/async-session-r2d2) - sqlite only
 
 ## Framework implementations