update the showcase instructions to be more specific

Summary:
I updated the showcase instructions during the docdown but after getting some more requests I think these should be more specific.

Two things in particular. It isn't very useful to link to some generic page on the company's own website, especially if the company has zero news coverage. Secondly, this content should be in English because that's what most readers know.

So I just updated the instructions to say this.
Closes https://github.com/facebook/react-native/pull/8788

Differential Revision: D3564132

Pulled By: JoelMarcey

fbshipit-source-id: d75adda0d2161e795436d0075b814a40c69383a3