Adds changelog file.

Why:

* To keep what's changing for future reference.

Author: jakubpawlowicz

History.md

@@ -0,0 +1,5 @@
+4.0.0 / 20xx-xx-xx
+==================
+
+* Initial release of separate clean-css-cli.
+* See [clean-css release notes](https://github.com/jakubpawlowicz/clean-css/blob/master/History.md#400--2017-01-23) for a full list of changes.