docs(techniques/mongo): added sessions section

added a small section on how to start a new session using the connection object

Author: ahmedwael216

content/techniques/mongo.md

@@ -191,6 +191,14 @@ import { Connection } from 'mongoose';
 export class CatsService {
   constructor(@InjectConnection() private connection: Connection) {}
 }
+```
+### Sessions
+To start a session with mongoose you shouldn't use `mongoose.startSession()` directly. Instead inject the connection using [@InjectConnection](http://localhost:4200/techniques/mongodb#connection), then use the connection object to start the session.
+
+```typescript
+const session = await this.connection.startSession();
+session.startTransaction();
+
 ```
 
 #### Multiple databases