Summary:
The tutorial has moved to a different URL, so the link currently ends up at the getting started page.

I also changed the remaining non-HTTPS urls in README.md to use HTTPS after checking they supported it. And I changed the Addons.Mozilla.org link to a language-agnostic one.
Closes https://github.com/facebook/react-native/pull/8047

Differential Revision: D3430542

Pulled By: javache

fbshipit-source-id: 4c5ccea9a5720126870a5df1be5073a6cc2f34ae