Added 'Thank you for contributing in Vietnamese

[roonyh]

Thanks! It is important that we have the correct translation rather than the simple Google or another machine's, possibly not 100% correct, translation. Do you speak Vietnamese? If not could you please get someone who does to confirm the correctness of this translation?

Also a rebase would be nice. http://stackoverflow.com/questions/17182624/contributing-to-project-on-github-how-to-rebase-my-pull-request-on-top-of-mast

[tradzik]

Closing, as we're not sure about correctness of translation + no reaction from author.

[hongquan]

I confirm that the translation is correct.

[tradzik]

@hongquan Thanks!