add CHANGELOG

CHANGELOG.md

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+# Changelog: langtest_V0.50 -> HEAD
+
+### Table of contents
+- Current [HEAD](#user-content-head)
+- Release [langtest_V0.50](#user-content-langtest_V0.50)
+
+<!-- --------------------------------------------------------------------- -->
+---
+## HEAD <a name="head"></a>
+### General Proviso
+The HEAD version shows the current development. No guarantees that software or
+the documentation is consistent.
+
+### Summary
+
+<!-- --------------------------------------------------------------------- -->
+---
+## 2017-12-30: [langtest_V0.50](https://github.com/wfjm/mvs38j-langtest/releases/tag/V0.55) - rev 980(wfjm) <a name="langtest_V0.50"></a>
+
+### Summary
+- first release, announced in [turnkey-mvs](https://groups.yahoo.com/neo/groups/turnkey-mvs/conversations/messages/10795)