Update changelog.md

changelog.md

+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+## [3.1] - 2021-05-25
+- Update Junos Netconf backend to use configurable virtual circuit ID prefixes.
+
+## [3.0] - 2021-05-10
+- initial setup of GCS specific OpenNSA repository