Docs - README.md - tweaks (separator, thumbnail logo)

Summary:
**this PR does basically aim at adding the following tweaks to README.md file:**
- *separators*
- *thumbnail logo*

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**main motivation behind such:**
- *improving overall document accessebility*

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<img src="https://avatars2.githubusercontent.com/u/69631?s=200&v=4" width="50"></img> <img src="https://avatars1.githubusercontent.com/u/7303598?s=460&v=4" width="50"></img>
Closes #16509

Differential Revision: D6465270

Pulled By: hramos

fbshipit-source-id: 0443765bdc0001926157ad0443a035be6992c418

Author: tiagomoraismorgado

README.md

@@ -20,6 +20,8 @@ Supported operating systems are >= Android 4.1 (API 16) and >= iOS 8.0.
 - [Contributing](#contributing)
 - [License](#license)
 
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 ## Getting Started
 
 Follow the [Getting Started guide](https://facebook.github.io/react-native/docs/getting-started.html). The recommended way to install React Native depends on your project. Here you can find short guides for the most common scenarios:
@@ -28,6 +30,8 @@ Follow the [Getting Started guide](https://facebook.github.io/react-native/docs/
 - [Creating a New Application](http://facebook.github.io/react-native/docs/getting-started.html)
 - [Adding React Native to an Existing Application](http://facebook.github.io/react-native/docs/integration-with-existing-apps.html)
 
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 ## Documentation
 
 [The website’s documentation](https://facebook.github.io/react-native/docs/getting-started.html) is divided into multiple sections.
@@ -40,10 +44,14 @@ Another great way to learn more about the components and APIs included with Reac
 
 The React Native documentation only discusses the components, APIs and topics specific to React Native (React on iOS and Android). For further documentation on the React API that is shared between React Native and React DOM, refer to the [React documentation](https://facebook.github.io/react/).
 
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 ## Upgrading
 
 React Native is under active development. See the guide on [upgrading React Native](https://facebook.github.io/react-native/docs/upgrading.html) to keep your project up-to-date.
 
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 ## Contributing
 
 Read below to learn how you can take part in improving React Native.
@@ -60,8 +68,14 @@ Read our [contributing guide](https://facebook.github.io/react-native/docs/contr
 
 We have a list of [beginner friendly issues](https://github.com/facebook/react-native/labels/Good%20First%20Issue) to help you get your feet wet in the React Native codebase and familiar with our contribution process. This is a great place to get started.
 
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 ## License
 
 React Native is [BSD licensed](./LICENSE). We also provide an additional [patent grant](./PATENTS).
 
 React Native documentation is [Creative Commons licensed](./LICENSE-docs).
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+---
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+<img src="https://avatars2.githubusercontent.com/u/69631?s=200&v=4" width="50"></img>