Add information about building for VS14 and VS15 to CONTRIBUTING.md
Add information about building for VS14 and VS15 to CONTRIBUTING.md

AlexanderSher/RTVS,karthiknadig/RTVS,AlexanderSher/RTVS,karthiknadig/RTVS,karthiknadig/RTVS,AlexanderSher/RTVS,MikhailArkhipov/RTVS,MikhailArkhipov/RTVS,AlexanderSher/RTVS,karthiknadig/RTVS,AlexanderSher/RTVS,karthiknadig/RTVS,AlexanderSher/RTVS,MikhailArkhipov/RTVS,AlexanderSher/RTVS,MikhailArkhipov/RTVS,karthiknadig/RTVS,MikhailArkhipov/RTVS,karthiknadig/RTVS,MikhailArkhipov/RTVS,MikhailArkhipov/RTVS