Add links in README

[vasylnahuliak]

To increase the popularity of the library, add the following links to the documentation:

• https://reactnative.directory/?search=react-native-braintree
• https://reintech.io/blog/tutorial-for-react-developers-what-are-the-best-react-native-libraries-for-creating-mobile-payments
• https://reactnativeexample.com/a-react-native-interface-for-integrating-payments-using-braintree/
• https://www.npmjs.com/package/@ekreative/react-native-braintree

TODO:

• [x] come up with a name for the section
• [x] add a minimal description

[Hollister009]

@vasylnahuliak can you assign this one to me?

[Hollister009]

• [x] come up with a name for the section: Useful links and resources

[Hollister009]

Create PR, please review: https://github.com/ekreative/react-native-braintree/pull/64

[vasylnahuliak]

Awesome!