docs(configuration): add redis as session storage

docs/sources/installation/configuration.md

@@ -373,7 +373,7 @@ Set to `true` to enable auto sign up of users who do not exist in Grafana DB. De
 
 ### provider
 
-Valid values are `memory`, `file`, `mysql`, `postgres`, `memcache`. Default is `file`.
+Valid values are `memory`, `file`, `mysql`, `postgres`, `memcache` or `redis`. Default is `file`.
 
 ### provider_config
 
@@ -384,6 +384,7 @@ session provider you have configured.
 - **mysql:** go-sql-driver/mysql dsn config string, e.g. `user:password@tcp(127.0.0.1:3306)/database_name`
 - **postgres:** ex:  user=a password=b host=localhost port=5432 dbname=c sslmode=disable
 - **memcache:** ex:  127.0.0.1:11211
+- **redis:** ex: `addr=127.0.0.1:6379,pool_size=100,db=grafana`
 
 If you use MySQL or Postgres as the session store you need to create the
 session table manually.