GSoC20 start contribution in React Native codebase

The BUILD.md need to be updated only for Building the Mobile application
All the library is inside ReactNativeClient/lib folder
so when building the mobile application in doc it is mentioned that
we are required to follow the below commands as per the doc

npm run tsc
cd ReactNativeClient
npm install
react-native run-ios
# Or: react-native run-android

but running

npm run tsc 

without

npm run copyLib

creates problem

but after running

npm run copyLib

then

npm run tsc 

works perfectly

Please take a look