README.md: Describe more config variables, cleanup

Added some documentation of environment variables to README.md,
specifically WORK_DIR and DEPLOY_DIR.  Explicitly did not include
SCRIPT_DIR at this time because that appears to require some small work
before it can be changed.

Took the chance for some other syntax cleanups while I was at it, mostly
breaking long lines and inserting whitespace where some Markdown parsers
require it.

README.md

@@ -1,12 +1,24 @@
 # pi-gen
+
 _Tool used to create the raspberrypi.org Raspbian images_
 
+
 ### TODO
 1. Documentation
 
+
 ## Dependencies
 
-`quilt parted realpath qemu-user-static debootstrap zerofree pxz zip dosfstools bsdtar libcap2-bin grep rsync`
+On Debian-based systems:
+
+```bash
+apt-get install quilt parted realpath qemu-user-static debootstrap zerofree pxz zip \
+dosfstools bsdtar libcap2-bin grep rsync
+```
+
+The file `depends` contains a list of tools needed.  The format of this
+package is `<tool>[:<debian-package>]`.
+
 
 ## Config
 
@@ -29,29 +41,57 @@ The following environment variables are supported:
    will not be included in the image, making it safe to use an `apt-cacher` or
    similar package for development.
 
+ * `BASE_DIR`  (Default: location of `build.sh`)
+
+   **CAUTION**: Currently, changing this value will probably break build.sh
+
+   Top-level directory for `pi-gen`.  Contains stage directories, build
+   scripts, and by default both work and deployment directories.
+
+ * `WORK_DIR`  (Default: `"$BASE_DIR/work"`)
+
+   Directory in which `pi-gen` builds the target system.  This value can be
+   changed if you have a suitably large, fast storage location for stages to
+   be built and cached.  Note, `WORK_DIR` stores a complete copy of the target
+   system for each build stage, amounting to tens of gigabytes in the case of
+   Raspbian.
+
+ * `DEPLOY_DIR`  (Default: `"$BASE_DIR/deploy"`)
+
+   Output directory for target system images and NOOBS bundles.
+
+
 A simple example for building Raspbian:
 
 ```bash
 IMG_NAME='Raspbian'
 ```
 
+
 ## Docker Build
 
 ```bash
 vi config         # Edit your config file. See above.
 ./build-docker.sh
 ```
+
 If everything goes well, your finished image will be in the `deploy/` folder.
 You can then remove the build container with `docker rm pigen_work`
 
 If something breaks along the line, you can edit the corresponding scripts, and
 continue:
 
-```
+```bash
 CONTINUE=1 ./build-docker.sh
 ```
 
-There is a possibility that even when running from a docker container, the installation of `qemu-user-static` will silently fail when building the image because `binfmt-support` _must be enabled on the underlying kernel_. An easy fix is to ensure `binfmt-support` is installed on the host machine before starting the `./build-docker.sh` script (or using your own docker build solution).
+There is a possibility that even when running from a docker container, the
+installation of `qemu-user-static` will silently fail when building the image
+because `binfmt-support` _must be enabled on the underlying kernel_. An easy
+fix is to ensure `binfmt-support` is installed on the host machine before
+starting the `./build-docker.sh` script (or using your own docker build
+solution).
+
 
 ## Stage Anatomy
 
@@ -104,14 +144,22 @@ maintenance and allows for more easy customization.
    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).
+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).
+
+```bash
 # Example for building a lite system
-$ touch ./stage3/SKIP ./stage4/SKIP
-$ rm stage4/EXPORT*
+touch ./stage3/SKIP ./stage4/SKIP
+rm stage4/EXPORT*
 ```
-If you wish to build further configurations upon (for example) the lite system, you can also delete the contents of `./stage3` and `./stage4` and replace with your own contents in the same format.
+
+If you wish to build further configurations upon (for example) the lite
+system, you can also delete the contents of `./stage3` and `./stage4` and
+replace with your own contents in the same format.