Update installation instructions now that React Native supports autolinking

[pcowgill]

Now that React Native supports autolinking, will the installation instructions for this package change at all?

https://github.com/react-native-community/cli/blob/master/docs/autolinking.md

Thanks!

[mvayngrib]

@pcowgill yes, for RN >= 0.60, react-native link react-native-randombytes is no longer necessary