CONTRIBUTING: add instructions to contribute

This doc is shown to people who open a pull request on Github
as described at https://github.com/blog/1184-contributing-guidelines
README: Use correct upstream build status for travis badge

@rismoney transferred the upstream repo to the chocolatey org
a few months ago. It makes sense that the build status should
be badged against the upstream org.
spec: protect against trailing whitespace

Add unit test and convert README with dos2unix line endings
to make the test pass.

    git commits
      should not introduce trailing whitespace