Improve Visual Studio instructions.

Provide the cmake command lines for Visual Studio 2017 and 2019, for
both the x64 and 32-bit x86 build targets.

Change-Id: I2af7349d3d8bf0ee88f18ea9209178c26554590f

README.md

diff --git a/README.md b/README.md
index c93a17b..cf057ae 100644
--- a/README.md
+++ b/README.md
@@ -219,8 +219,23 @@
 
 ~~~
     # This does not require a bash shell; Command Prompt (cmd.exe) is fine.
+    # This assumes the build host is a Windows x64 computer.
+
+    # To build with Visual Studio 2019 for the x64 target:
     $ cmake path/to/aom -G "Visual Studio 16 2019"
     $ cmake --build .
+
+    # To build with Visual Studio 2019 for the 32-bit x86 target:
+    $ cmake path/to/aom -G "Visual Studio 16 2019" -A Win32
+    $ cmake --build .
+
+    # To build with Visual Studio 2017 for the x64 target:
+    $ cmake path/to/aom -G "Visual Studio 15 2017" -T host=x64 -A x64
+    $ cmake --build .
+
+    # To build with Visual Studio 2017 for the 32-bit x86 target:
+    $ cmake path/to/aom -G "Visual Studio 15 2017" -T host=x64
+    $ cmake --build .
 ~~~
 
 NOTE: The build system targets Windows 7 or later by compiling files with