Update and add links in the README
Update and add links in the README

Some of the links were prefixed with the repo URL. They are now relative links.

The list of available methods now also links to the method details.

leoilab/react-native-analytics-segment-io,leoilab/react-native-analytics-segment-io,leoilab/react-native-analytics-segment-io,leoilab/react-native-analytics-segment-io