Switch changelog format to single CHANGELOG.md in the root directory of the repo (#397)

Author: traversaro

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## [Unreleased]
+
+### Changed
+* Releases after 0.13.2 document their changes in a `CHANGELOG.md` file in the root of the repo (https://github.com/robotology/ycm/pull/397).

help/release/0.13.2.rst

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+YCM 0.13.2 (YYYY-MM-DD) Release Notes
+*************************************
+
+.. only:: html
+
+  .. contents::
+
+Changes made since YCM 0.13.1 include the following.
+
+Important Changes
+=================
+
+* For releases after YCM 0.13.2, the changelog is now mantained in the `CHANGELOG.md` file in the root of the source code.