bpo-31691: Specify how to build installer in Windows (#4430)

PCbuild/readme.txt

@@ -40,13 +40,20 @@ MSVCRT90.dll.
 
 For other Windows platforms and compilers, see ../PC/readme.txt.
 
-All you need to do to build is open the solution "pcbuild.sln" in Visual
-Studio, select the desired combination of configuration and platform,
+To build modules that depend on external libraries, you need to download
+(and, for some of them, build) those first. It's thus recommended to build
+from the command line once as specified below under "Getting External Sources"
+as that does this automatically.
+
+Then, to continue development, you can open the solution "pcbuild.sln" in
+Visual Studio, select the desired combination of configuration and platform,
 then build with "Build Solution".  You can also build from the command
 line using the "build.bat" script in this directory; see below for
 details.  The solution is configured to build the projects in the correct
 order.
 
+To build an installer package, refer to the README in the Tools/msi folder.
+
 The solution currently supports two platforms.  The Win32 platform is
 used to build standard x86-compatible 32-bit binaries, output into this
 directory.  The x64 platform is used for building 64-bit AMD64 (aka