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 https://github.com/facebook/react-native/pull/16509

Differential Revision: D6465270

Pulled By: hramos

fbshipit-source-id: 0443765bdc0001926157ad0443a035be6992c418