Improve documentation
Improve documentation

- Cleaned up language and formatting
- Added information about known limitations
- Added details about element parameters
- Changed example code to use ES2015 imports

dmazin/react-native-router-flux,Swordsman-Inaction/react-native-router-flux,peteychuk/react-native-router-flux,dmazin/react-native-router-flux,dmazin/react-native-router-flux,Wondermall/react-native-router-flux,aksonov/react-native-router-flux,gectorat/react-native-router-flux,Swordsman-Inaction/react-native-router-flux,hungtn/react-native-router-flux,mcabs3/react-native-router-flux,charpeni/react-native-router-flux,charpeni/react-native-router-flux,mcabs3/react-native-router-flux,gectorat/react-native-router-flux,gijoehosaphat/react-native-router-flux,Swordsman-Inaction/react-native-router-flux,r0b1n/react-native-router-flux,r0b1n/react-native-router-flux,hungtn/react-native-router-flux,charpeni/react-native-router-flux,Wondermall/react-native-router-flux,mcabs3/react-native-router-flux,gijoehosaphat/react-native-router-flux,gectorat/react-native-router-flux,peteychuk/react-native-router-flux,gijoehosaphat/react-native-router-flux,Wondermall/react-native-router-flux,peteychuk/react-native-router-flux