Make Visual Studio 2017 minimum supported version.

Document that Visual Studio 2017 is the minimum supported version of
Visual Studio. Update the example commands to build with Visual Studio.

Change-Id: I04fed546693d8b370f2f6f18ba0a5c1394377d4f
diff --git a/README.md b/README.md
index 6fea7b0..c93a17b 100644
--- a/README.md
+++ b/README.md
@@ -214,12 +214,13 @@
 ### Microsoft Visual Studio builds
 
 Building the AV1 codec library in Microsoft Visual Studio is supported. Visual
-Studio 2015 (14.0) or later is required. The following example demonstrates
+Studio 2017 (15.0) or later is required. The following example demonstrates
 generating projects and a solution for the Microsoft IDE:
 
 ~~~
-    # This does not require a bash shell; command.exe is fine.
-    $ cmake path/to/aom -G "Visual Studio 15 2017"
+    # This does not require a bash shell; Command Prompt (cmd.exe) is fine.
+    $ cmake path/to/aom -G "Visual Studio 16 2019"
+    $ cmake --build .
 ~~~
 
 NOTE: The build system targets Windows 7 or later by compiling files with