My first contribution to the documentation

[Juanjo Marín <juanjomarin96 yahoo es>]

Hi !

Hi, I'm Juanjo Marin, I'm member of the a11y team and I'd like to help to maintain the a11y documentation. This is my first humble addition. It's not a fix, it just an addition. It a key navigation suggestion of Meta+down-arrow as complement to Meta+up-arrow. Though Meta+down-arrow is mentioned in the document, I think it is useful to mentioned in the document, I think is better to comment as a natural combination in the key navigation workflow.

If you agree that it is a worth addition and the patch is good enough, I can push the patch.

Thanks !,

    -- Juanjo Marin