update / create Social Share for GDG DevFest Seattle 2017

[cliveb]

Please make new social share image
https://github.com/GDGSeattle/hoverboard/blob/develop/images/social-share.jpg

When someone shares Hoverboard site on social media (say twitter/ facebook). The social share image card defaults not the site thumbnail. Last year I deleted the social card share. But the site social media was a goofy share. I think this is how it works (not 100%).

[cliveb]

@margaretmz If can't get to this let me know the text and back ground. I will ask my sister to fix it up.