Fixing bad link to RN documentation.

Summary:
The link referencing the root of RN documentation was leading to a 404 error. As the docs don't seem to have a true "index" page, it makes sense that it goes to the top level entry in the docs, the "Getting Started Section". The alternative solution may be not linking that phrase at all, this seems helpful though.

No changes in this pull request require any sort of unit/AB testing, the link directing to the correct place should be apparent.

[DOCS] [BUGFIX] [README.md] - Fixing doc resource link.
Closes facebook#17048

Differential Revision: D6464455

Pulled By: hramos

fbshipit-source-id: 710dc882672d762478910538341b57cb1bb129bb

README.md

@@ -30,7 +30,7 @@ Follow the [Getting Started guide](https://facebook.github.io/react-native/docs/
 
 ## Documentation
 
-[The website’s documentation](https://facebook.github.io/react-native/docs/) is divided into multiple sections.
+[The website’s documentation](https://facebook.github.io/react-native/docs/getting-started.html) is divided into multiple sections.
 - There are **Guides** that discuss topics like [debugging](https://facebook.github.io/react-native/docs/debugging.html), [integrating with existing apps](https://facebook.github.io/react-native/docs/integration-with-existing-apps.html), and [the gesture responder system](https://facebook.github.io/react-native/docs/gesture-responder-system.html).
 - The **Components** section covers React components such as [`View`](https://facebook.github.io/react-native/docs/view.html) and [`Button`](https://facebook.github.io/react-native/docs/button.html).
 - The **APIs** section covers other libraries like [Animated](https://facebook.github.io/react-native/docs/animated.html) and [StyleSheet](https://facebook.github.io/react-native/docs/stylesheet.html) that aren’t React components themselves.