Adopt new format for changelog

From now on the changelog will adhere to the https://keepachangelog.com/en/1.0.0/ format

CHANGELOG

-============
-v 0.3.1
-============
-Marc Vef:
- 2018-03-04 Read-write process improved, adafs version to 0.3.1 with improved Filemap

CHANGELOG.md

+# Changelog
+All notable changes to GekkoFS project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [0.3.1] - 2018-03-04
+### Changed
+- Read-write process improved. @Marc vef
+- Improved Filemap. @Marc Vef