adding documentation

Author: Gabriel Mendes

README.md

@@ -25,6 +25,7 @@ To create new injectors for your topics, you should create a new kubernetes depl
 - `ELASTICSEARCH_PASSWORD` Elasticsearch password. **OPTIONAL**
 - `ELASTICSEARCH_SCHEME` scheme to be used when connecting to elasticsearch(http or https). Defaults to http. **OPTIONAL**
 - `ELASTICSEARCH_IGNORE_CERT` if set to "true", ignores certificates when connecting to a secure elasticsearch cluster. Defaults to false. **OPTIONAL**
+- `ELASTICSEARCH_DISABLE_SNIFFING` if set to "true", the client will not sniff Elasticsearch nodes during the node discovery process. Defaults to false. **OPTIONAL**
 - `KAFKA_CONSUMER_CONCURRENCY` Number of parallel goroutines working as a consumer. Default value is 1 **OPTIONAL**
 - `KAFKA_CONSUMER_BATCH_SIZE` Number of records to accumulate before sending them to elasticsearch(for each goroutine). Default value is 100 **OPTIONAL**
 - `ES_INDEX_COLUMN` Record field to append to index name. Ex: to create one ES index per campaign, use "campaign_id" here **OPTIONAL**