README.md: Expand the acronym to help readers understand (#511)

README.md

@@ -14,10 +14,13 @@ Package](https://img.shields.io/pypi/v/parsedmarc.svg)](https://pypi.org/project
 
 `parsedmarc` is a Python module and CLI utility for parsing DMARC
 reports. When used with Elasticsearch and Kibana (or Splunk), it works
-as a self-hosted open source alternative to commercial DMARC report
+as a self-hosted open-source alternative to commercial DMARC report
 processing services such as Agari Brand Protection, Dmarcian, OnDMARC,
 ProofPoint Email Fraud Defense, and Valimail.
 
+> [!NOTE]
+> __Domain-based Message Authentication, Reporting, and Conformance__ (DMARC) is an email authentication protocol.
+
 ## Help Wanted
 
 This project is maintained by one developer. Please consider