build: clarify instructions for Windows

Clarified up front that Python must be installed for all Windows builds
and it must be in the path. Simplified the Windows instructions by
removing the redundant msbuild guidance.

Also added subheadings for each unique build environment.

README.md

@@ -62,31 +62,40 @@ To build with autotools:
     $ make check
     $ make install
 
-To build with Visual Studio run the vcbuild.bat file which will
-checkout the GYP code into build/gyp and generate the uv.sln and
-related files.
+### Windows
 
-Windows users can also build from the command line using msbuild.
-This is done by running vcbuild.bat from Visual Studio command prompt.
+First, Python 2.6 or 2.7 must be installed as it is required by [GYP][].
 
-To have GYP generate build script for another system, make sure that
-you have Python 2.6 or 2.7 installed, then checkout GYP into the
+Also, the directory for the preferred Python executable must be specified
+by the `PYTHON` or `Path` environment variables.
+
+To build with Visual Studio, launch a git shell (e.g. Cmd or PowerShell)
+and run vcbuild.bat which will checkout the GYP code into build/gyp and
+generate uv.sln as well as related project files.
+
+To have GYP generate build script for another system, checkout GYP into the
 project tree manually:
 
     $ mkdir -p build
     $ git clone https://git.chromium.org/external/gyp.git build/gyp
 
-Unix users run:
+### Unix
+
+Run:
 
     $ ./gyp_uv -f make
     $ make -C out
 
-Macintosh users run:
+### OS X
+
+Run:
 
     $ ./gyp_uv -f xcode
     $ xcodebuild -project uv.xcodeproj -configuration Release -target All
 
-To build for android:
+### Android
+
+Run:
 
     $ source ./android-configure NDK_PATH gyp
     $ make -C out
@@ -103,7 +112,7 @@ Studio license.
 
 Linux using the GCC toolchain.
 
-MacOS using the GCC or XCode toolchain.
+OS X using the GCC or XCode toolchain.
 
 Solaris 121 and later using GCC toolchain.