docs(README): add ES_BULK_BACKOFF explanation

Author: racevedoo

README.md

@@ -29,6 +29,7 @@ To create new injectors for your topics, you should create a new kubernetes depl
 - `LOG_LEVEL` Determines the log level for the app. Should be set to DEBUG, WARN, NONE or INFO. Defaults to INFO. **OPTIONAL**
 - `METRICS_PORT` Port to export app metrics **REQUIRED**
 - `ES_BULK_TIMEOUT` Timeout for elasticsearch bulk writes in the format of golang's `time.ParseDuration`. Default value is 1s **OPTIONAL**
+- `ES_BULK_BACKOFF` Constant backoff when elasticsearch is overloaded. in the format of golang's `time.ParseDuration`. Default value is 1s **OPTIONAL**
 - `KAFKA_CONSUMER_RECORD_TYPE` Kafka record type. Should be set to "avro" or "json". Defaults to avro. **OPTIONAL**
 - `KAFKA_CONSUMER_METRICS_UPDATE_INTERVAL` The interval which the app updates the exported metrics in the format of golang's `time.ParseDuration`. Defaults to 30s. **OPTIONAL**