improve windows build instructions

INSTALL.md

@@ -94,6 +94,7 @@ on experience with the tools involved.
 ### Building using CMake (RECOMMENDED)
 
 Builds gRPC C and C++ with boringssl.
+- Install Visual Studio 2015 or 2017 (Visual C++ compiler will be used).
 - Install [CMake](https://cmake.org/download/).
 - Install [Active State Perl](https://www.activestate.com/activeperl/) (`choco install activeperl`)
 - Install [Ninja](https://ninja-build.org/) (`choco install ninja`)
@@ -101,7 +102,9 @@ Builds gRPC C and C++ with boringssl.
 - Install [yasm](http://yasm.tortall.net/) and add it to `PATH` (`choco install yasm`)
 - Run these commands in the repo root directory
 
-Using Ninja (faster build, supports boringssl's assembly optimizations)
+#### cmake: Using Ninja (faster build, supports boringssl's assembly optimizations).
+Please note that when using Ninja, you'll still need Visual C++ (part of Visual Studio)
+installed to be able to compile the C/C++ sources.
 ```
 > md .build
 > cd .build
@@ -110,7 +113,12 @@ Using Ninja (faster build, supports boringssl's assembly optimizations)
 > cmake --build .
 ```
 
-Using Visual Studio 2015 (can only build with OPENSSL_NO_ASM)
+#### cmake: Using Visual Studio 2015 (can only build with OPENSSL_NO_ASM).
+When using the "Visual Studio" generator,
+cmake will generate a solution (`grpc.sln`) that contains a VS project for 
+every target defined in `CMakeLists.txt` (+ few extra convenience projects
+added automatically by cmake). After opening the solution with Visual Studio 
+you will be able to browse and build the code as usual.
 ```
 > md .build
 > cd .build