Get better README.md

[dhamanutd]

I think there need some improvement to add more detail about How To Contribute. As i experienced when trying to contribute and run the app in android, it turns out i need to do some edit in android/app/build.gradle to make react-native run-android working properly.

I think to make this Open Source more friendly to contributors, we need to update README.md more detailed.

[terrysahaidak]

Hi @dhamanutd,

Thank you for your issues.
Feel free to make PR with these changes.
We'll appreciate that.