Add release notes document

[traversaro]

To better document the new releases, we should add and maintain a releases notes/change log document to the repo.
As I worked a bit with the multi-file structure of the release notes in YARP ( https://github.com/robotology/yarp/tree/master/doc/release ) and iDynTree ( https://github.com/robotology/idyntree/tree/master/doc/releases ), I think that having a separate file for each release add quite a bit of overhead for each release, especially if the release is done via the the GitHub's web interface. For this reason, for this repo I would like to try to use a single document for all the release notes, with different section for each release, something like what is documented in https://keepachangelog.com/en/1.0.0/ .

[traversaro]

Example of the format for the next feature release:

[3.2.0] - 2019-06-24

Added

• Add resetSimulation method to reset simulation state and time time, in the gazebo_yarp_clock RPC interface (Added Reset Simulation #345).
• Add resetSimulationState method to reset simulation state, but not the time, in the gazebo_yarp_clock RPC interface (Add resetSimulationState and regenerate thrift-generated files #424).
• Add gazebo_yarp_doublelaser plugin to simulate a double laser device (Double laser plugin #419).
• Add gazebo_yarp_configurationoverride plugin that can be attached to a model in order to override the sdf configuration of any other plugin that is attached to the same model (Add configurationOverride Plugin #401). This plugin is particularly useful to set the initial configuration of a given model, and is documented in https://github.com/robotology/gazebo-yarp-plugins/blob/v3.2.0/plugins/configurationoverride/README.md .
• Add features in gazebo_yarp_externalwrench to apply multiple external wrenchs at the same time (Update external wrench plugin with mutiple wrenches and improve rpc responses #418) .
• Add the yarpConfigurationString to configure the YARP plugins with a single yarpConfigurationString embedded in the SDF code of the plugin (Add <yarpConfigurationString> parameter #396).

Fixed

• Fix invalid camera parameters in gazebo_depthCamera plugin (Fix invalid camera parameters in gazebo_depthCamera plugin #408).

[traversaro]

Done in #432 .