* improve changelog.rst & developers/environment.rst

instructions on environment.rst less platform specific, can now be pasted&copied as is

re changelog.rst: should the other npm-releases appear in changelog.rst, too?
(0.6.4 - 0.6.12, via `npm view buster`)

re which branch to send pull reqs to: plz see ["buster-docs: pull requests for branch master or rather develop only?" on dev mailing list](https://groups.google.com/forum/#!topic/busterjs-dev/mnJGs92rY_8)