Add initial changelog for installer downloader

Author: faern

installer-downloader/CHANGELOG.md

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+# Changelog
+All notable changes are recorded here.
+
+### Format
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
+
+Entries should have the imperative form, just like commit messages. Start each entry with words like
+add, fix, increase, force etc.. Not added, fixed, increased, forced etc.
+
+Line wrap the file at 100 chars.                                              That is over here -> |
+
+### Categories each change fall into
+
+* **Added**: for new features.
+* **Changed**: for changes in existing functionality.
+* **Deprecated**: for soon-to-be removed features.
+* **Removed**: for now removed features.
+* **Fixed**: for any bug fixes.
+* **Security**: in case of vulnerabilities.
+
+## [Unreleased]
+- Initial support for downloading and installing the latest version of the Mullvad VPN app on
+  Windows and macOS.