Added placeholderStyle to Picker
Added placeholderStyle to Picker

Documentation added for issue fix [#1414](https://github.com/GeekyAnts/NativeBase/pull/1414) - Added placeholderStyle to Picker
Issue [#1413](https://github.com/GeekyAnts/NativeBase/issues/1413) - Add style placeholder
GeekyAnts/native-base-docs,GeekyAnts/native-base-docs