Add changelog for stix-shifter-threatbus

apps/stix-shifter/CHANGELOG.md

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+# Changelog
+
+This changelog documents all notable user-facing changes of
+`stix-shifter-threatbus`.
+
+Every entry has a category for which we use the following visual abbreviations:
+
+- 🎁 Features
+- 🧬 Experimental Features
+- ⚠️ Changes
+- ⚡️ Breaking Changes
+- 🐞 Bug Fixes
+
+## Unreleased
+
+- 🎁 `stix-shifter-threatbus` has come to life. This stand-alone application
+  connects to Threat Bus via ZeroMQ and bridges the gap between Threat Bus and
+  commercial security tools, like
+  [IBM QRadar](https://www.ibm.com/security/security-intelligence/qradar) or
+  [Splunk](https://www.splunk.com/). `stix-shifter-threatbus` uses
+  [STIX-Shifter](https://github.com/opencybersecurityalliance/stix-shifter) to
+  first translate STIX-2 Indicators to native queries for commercial tools and
+  then execute these queries to log the results.
+  [#118](https://github.com/tenzir/threatbus/pull/118)