Add machine readable status file

[matthias-reis]

In order to build a software to keep track of the changes and releases, the release process should create a file containing entries for versions with the related released packages.

Example:

{
  "1.0.0": {
    "date": "2017-03-28T07:09:51.230Z",
    "packages": {
      "fancy-package": {
        "bump": "minor",
        "version": "0.4.0"
      },
     "shiny-tool": {
        "bump": "major",
        "version": "2.0.0"
      }
  }
}

This file will be extended every time a build is releases and shoud be part of the git repo.