Google's Gary Illyes has confirmed that Google will be publishing better recommendations for webmasters around the mobile-first index. He said this yesterday on Twitter:
@s_matharu yes— Gary Illyes ᕕ( ᐛ )ᕗ (@methode) November 22, 2016
What is happening now, as far as I believe, is that Google is testing the mobile-first index on a subset of users. They are tracking issues and corner cases and trying to see how best to adjust things so that when it does fully roll out that it doesn't have a huge impact on the current search results. As Google finalizes these plans and they are ready to roll it out fully, I suspect that blog post with recommendations will come.
It is very soon - very soon and I suspect it won't fully roll out until the end of Q1 2017.
Forum discussion at Twitter.