Update CHANGELOG for #48

Author: gadomski

CHANGELOG.md

@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Added
+
+- `Client.open` falls back to the `STAC_URL` environment variable if no url is provided as an argument [#48](https://github.com/stac-utils/pystac-client/pull/48)
+
 ## [v0.1.1] - 2021-04-16
 
 ### Added