[General] Update Readme

[krizzu]

Update README to cover:

• Available API (Installation steps, linking)
• Example usages
• Contribution list

[Pushplaybang]

@krizzu - Readme could also use a "motivation" or "why".

I'm guessing that asyncStorage is not going to be part of core in the future? couldn't see this in the RN docs, would be great to include a note about this.

[krizzu]

Hey @Pushplaybang,

It's all part of lean core effort for React Native. I guess it'd be fine to add this info to readme.