documentation for publish workflow

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (https://github.com/symfony-cmf/CoreBundle/pull/59)
| Applies to    | -
| Fixed tickets | -

This documentation is for the not yet merged publish workflow refactoring.

This is going to conflict with #148 - i suggest i manually port the still relevant parts of #148 over to here once its merged.