[bld] update changelog format.

Using the guidelines from here:
https://gitlab.cern.ch/be-co-ht-documents/project-management-guidelines/-/blob/master/how-to-write-changelog.rst



Signed-off-by: Dimitris Lampridis <dimitris.lampridis@cern.ch>

CHANGELOG.rst

 ..
-  SPDX-License-Identifier: CC-BY-SA-4.0
+  SPDX-License-Identifier: CC0-1.0
 
   SPDX-FileCopyrightText: 2019-2020 CERN
 
 ==========
 Change Log
 ==========
-Format: `Keep a Changelog <https://keepachangelog.com/en/1.0.0/>`_
-Versioning: `Semantic Versioning <https://semver.org/spec/v2.0.0.html>`_
+- Format inspired by: `Keep a Changelog <https://keepachangelog.com/en/1.0.0/>`_
+- Versioning scheme follows: `Semantic Versioning <https://semver.org/spec/v2.0.0.html>`_
+
+Unreleased
+==========
+https://www.ohwr.org/project/general-cores/compare/master...proposed_master
 
-[Unreleased]
-============
 Added
 -----
 - [hdl] VHDL functions to convert characters and strings to upper/lower case.
-- [sw][i2c] support for kernel greater than 4.7
+- [sw][i2c] support for kernel greater than 4.7.
+
 Changed
 -------
 - [hdl] Rewritten the WB master interface used in simulations.
 
-[1.0.3]_ - 2020-01-15
-====================
+1.0.3 - 2020-01-15
+==================
+https://www.ohwr.org/project/general-cores/tags/v1.0.3
+
 Changed
 -----
 - [sw] add more file to .gitignore
 
-[1.0.2]_ - 2019-10-24
-====================
+1.0.2 - 2019-10-24
+==================
+https://www.ohwr.org/project/general-cores/tags/v1.0.2
+
 Fixed
 -----
 - [ci] forgot rule to publish RPMs
 
-[1.0.1]_ - 2019-10-24
-====================
+1.0.1 - 2019-10-24
+==================
+https://www.ohwr.org/project/general-cores/tags/v1.0.1
+
 Added
 -----
 - [ci] building and publish RPMs automatically on new releases
+
 Changed
 -------
 - [sw] Makefiles have been changed to better support RPM generation
 
-[1.0.0]_ - 2019-10-21
-====================
+1.0.0 - 2019-10-21
+==================
+https://www.ohwr.org/project/general-cores/tags/v1.0.0
+
 Added
 -----
 - First release of general-cores.
-
-.. _[Unreleased]: https://www.ohwr.org/project/general-cores/compare/v1.0.3...proposed_master
-
-.. _[1.0.3]: https://www.ohwr.org/project/general-cores/tags/v1.0.3
-.. _[1.0.2]: https://www.ohwr.org/project/general-cores/tags/v1.0.2
-.. _[1.0.1]: https://www.ohwr.org/project/general-cores/tags/v1.0.1
-.. _[1.0.0]: https://www.ohwr.org/project/general-cores/tags/v1.0.0