docs: add retention policy notes

docs/sources/configuration/direct_db_datasource.md

@@ -32,3 +32,10 @@ database name (usually, `zabbix`) and specify credentials.
 ### Security notes
 
 Make sure you use read-only user for Zabbix database.
+
+## InfluxDB
+
+Select _InfluxDB_ data source type and provide your InfluxDB instance host address and port (8086 is default). Fill
+database name you configured in the [effluence](https://github.com/i-ky/effluence) module config (usually, `zabbix`) and specify credentials.
+
+![Configure InfluxDB data source](../img/configuration-influxdb_ds_config.png)

docs/sources/img/configuration-influxdb_ds_config.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:216a5d1a495fbd2093446c67e09a5dc669c65f541e65fcc13802facd411b5434
+size 111917

docs/sources/reference/direct-db-connection.md

@@ -81,7 +81,13 @@ As you can see, the Grafana-Zabbix plugin uses aggregation by a given time inter
 ## InfluxDB
 Zabbix supports loadable modules which makes possible to write history data into an external database. There's a [module](https://github.com/i-ky/effluence) for InfluxDB written by [@i-ky](https://github.com/i-ky) which can export history into InfluxDB in real-time.
 
-InfluxDB Query Example:
+#### InfluxDB retention policy
+In order to keep database size under control, you should use InfluxDB retention policy mechanism. It's possible to create retention policy for long-term data and write aggregated data in the same manner as Zabbix does (trends). Then this retention policy can be used in plugin for getting data after a certain period ([Retention Policy](../../configuration/#direct-db-connection) option in data source config). Read more about how to configure retention policy for using with plugin in effluence module [docs](https://github.com/i-ky/effluence#database-sizing).
+
+#### InfluxDB Query
+
+Eventually, plugin generates InfluxDB query similar to this:
+
 ```sql
 SELECT MEAN("value")
 FROM "history"