Add CHANGELOG entry for #120

Author: duckontheweb

CHANGELOG.md

@@ -6,6 +6,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Fixed
+
+- `Client.get_collections` raised an exception when API did not publish `/collections` conformance class instead of falling back to using child links
+  [#120](https://github.com/stac-utils/pystac-client/pull/120)
+
 ## [0.3.0] - 2021-09-28
 
 ### Added