Update ruby-core page (en)

**(Not for merging)**

Changes:

* Update information to the 1.9/2.0 series.
* Some minor improvements.
* I tried to improve(?) the section on how to submit patches and lay more stress on the predominant role of ruby-trunk as basis for all patches.

The first sentence might not be too convincing, but _"with Ruby 2.0 underway"_ can't be kept.

@postmodern, @chikamichi, @JEG2 Feedback?

@hsbt Could especially you as one of the developers check whether the page makes sense in its current version? I had a look at some how-to pages on the issue tracker, but some of my changes are based more on assumptions than knowledge.