Add changelog files

[alixdamman]

@gdementen I wonder if it wouldn't be easier to make larray-editor project to have its own change.rst files and then to update the Releaser to copy/paste their content at the end of the change.rst files of the larray project ?

[gdementen]

Yes, indeed. I imagine something with several sections per version in the changelog: "Core", "Graphical editor (GUI)", "Eurostat", etc.