Add bare-bones CHANGELOG

CHANGELOG.md

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+# Changelog
+
+All notable changes to Rackstash will be documented in this file. This
+project adheres to [Semantic Versioning](http://semver.org/).
+
+## HEAD
+
+This is a complete rewrite of Rackstash. The basic ideas are retained but the design, the technical implementation, and some of the interfaces are new.
+
+We will attempt to provide a migration path from Rackstash 0.0.1 to 0.2 without too much pain.
+
+### Changes
+
+* Dropped support for Ruby < 2.1.0
+* Dropped support for Rails. See [rackstash-rails](https://github.com/meineerde/rackstash-rails) for the companion gem.
+
+* {Rackstash::Logger#<<} now emits a JSON document similar to all other logger methods. I will however add the eaw unformatted message to the `"message"` field.
+* Exception backtraces are not emitted in the `error_trace` field instead of `error_backtrace`. That way, the field is shown below the error and error message in Kibana which makes a much nicer experience.
+
+## [0.0.1]
+
+* The first version of the Rackstash gem