DOCS: add document describing the release policy and procedure

Related to #872. Not sure if the "release procedure" is complete.

I've also created a [release notes draft](https://gist.github.com/ghedo/1d157c54bdf3e591280c) for the 0.4.0 release. I tried to list the most visible user-facing changes, but I most likely missed a lot of things (particularly in the bug fixes section and for windows/mac os x related stuff), so suggestions are welcome.

I'd expect that with a release manager future releases will happen more frequently, so the release notes will probably be more manageable.