Add stage specification instructions

README.md

@@ -45,7 +45,7 @@ The build of Raspbian is divided up into several stages for logical clarity
 and modularity.  This causes some initial complexity, but it simplifies
 maintenance and allows for more easy customization.
 
- - Stage 0, bootstrap.  The primary purpose of this stage is to create a
+ - **Stage 0** - bootstrap.  The primary purpose of this stage is to create a
    usable filesystem.  This is accomplished largely through the use of
    `debootstrap`, which creates a minimal filesystem suitable for use as a
    base.tgz on Debian systems.  This stage also configures apt settings and
@@ -53,7 +53,7 @@ maintenance and allows for more easy customization.
    minimal core is installed but not configured, and the system will not quite
    boot yet.
 
- - Stage 1, truly minimal system.  This stage makes the system bootable by
+ - **Stage 1** - truly minimal system.  This stage makes the system bootable by
    installing system files like `/etc/fstab`, configures the bootloader, makes
    the network operable, and installs packages like raspi-config.  At this
    stage the system should boot to a local console from which you have the
@@ -62,7 +62,7 @@ maintenance and allows for more easy customization.
    really usable yet in a traditional sense yet.  Still, if you want minimal,
    this is minimal and the rest you could reasonably do yourself as sysadmin.
 
- - State 2, lite system.  This stage produces the Raspbian-Lite image.  It
+ - **Stage 2** - lite system.  This stage produces the Raspbian-Lite image.  It
    installs some optimized memory functions, sets timezone and charmap
    defaults, installs fake-hwclock and ntp, wifi and bluetooth support,
    dphys-swapfile, and other basics for managing the hardware.  It also
@@ -77,13 +77,24 @@ maintenance and allows for more easy customization.
    you were looking for something between truly minimal and Raspbian-lite,
    here's where you start trimming.
 
- - Stage 3, desktop system.  Here's where you get the full desktop system
+ - **Stage 3** - desktop system.  Here's where you get the full desktop system
    with X11 and LXDE, web browsers, git for development, Raspbian custom UI
    enhancements, etc.  This is a base desktop system, with some development
    tools installed.
 
- - Stage 4, complete Raspbian system.  More development tools, an email
+ - **Stage 4** - complete Raspbian system.  More development tools, an email
    client, learning tools like Scratch, specialized packages like sonic-pi and
    wolfram-engine, system documentation, office productivity, etc.  This is
    the stage that installs all of the things that make Raspbian friendly to
    new users.
+   
+### Stage specification
+If you wish to build up to a specified stage (such as building up to stage 2 for a lite system), place an empty file named `SKIP` in each of the `./stage` directories you wish not to include.
+
+Then remove the `EXPORT*` files from `./stage4` (if building up to stage 2) or from `./stage2` (if building a minimal system).
+
+```
+# Example for building a lite system
+$ touch ./stage3/SKIP ./stage4/SKIP
+$ rm stage4/EXPORT*
+```