adding OpenSearch support, fixing minor typos, and code styling (#481)

* adding OpenSearch support, fixing minor typos and code styling

* documentation update

docs/source/opensearch.md

@@ -0,0 +1,14 @@
+# OpenSearch and Grafana
+
+To set up visual dashboards of DMARC data, install OpenSearch and Grafana.
+
+## Installation
+
+OpenSearch: https://opensearch.org/docs/latest/install-and-configure/install-opensearch/index/
+Grafana: https://grafana.com/docs/grafana/latest/setup-grafana/installation/
+
+## Records retention
+
+Starting in version 5.0.0, `parsedmarc` stores data in a separate
+index for each day to make it easy to comply with records
+retention regulations such as GDPR.