Add changelog entry for logging functionality

We introduced basic logging in commit e69e4af but didn’t update the
changelog.  This patch adds the missing changelog entry.

CHANGELOG.md

@@ -3,6 +3,13 @@ SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
 SPDX-License-Identifier: MIT
 -->
 
+# Unreleased
+
+- Add basic logging using `env_logger` that can be enabled by setting the
+  environment variable `RUST_LOG=info`.
+  - Add `env_logger` dependency in version 0.7.1.
+  - Add `log` dependency in version 0.4.11.
+
 # v0.1.1 (2020-07-24)
 
 This patch release fixes some minor issues with the documentation displayed on