Format the possible version formats as a table

**Note I:** This syntax is supported by MarkdownExtra, which both getcomposer
and github are using.

**Note II:** This patch currently breaks the PDF, because pandoc does not like
non-standard markdown tables. Ideas for fixing this appreciated.

**Note III:** The idea for this patch came up a few weeks ago on IRC. We
agreed back then that a table would be a good idea.

**Note IIII:** This patch creates a stability section which opens the door for
finally documenting how stability works in the composer docs.

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With [this patch](https://github.com/composer/getcomposer.org/pull/53) applied to getcomposer, the result looks as follows:

![screen shot 2013-10-02 at 1 12 07 am](https://f.cloud.github.com/assets/88061/1250119/fa67eee2-2aee-11e3-88c3-073acc13c21e.png)