Prepare CHANGELOG.rst for upcoming changes

Author: lgrahl

CHANGELOG.rst

@@ -1,6 +1,11 @@
 Changelog
 *********
 
+`Unreleased`_ (YYYY-MM-DD)
+--------------------------
+
+- ...
+
 `3.0.4`_ (2017-05-23)
 ---------------------
 
@@ -11,5 +16,6 @@ Changelog
 
 - Initial publication on PyPI
 
+.. _Unreleased: https://github.com/lgrahl/threema-msgapi-sdk-python/compare/v3.0.4...HEAD
 .. _3.0.4: https://github.com/lgrahl/threema-msgapi-sdk-python/compare/v3.0.2...v3.0.4
 .. _3.0.2: https://github.com/lgrahl/threema-msgapi-sdk-python/compare/e982c74cbe564c76cc58322d3154916ee7f6863b...v3.0.2