Update GettingStarted.md with new environment vars

Change-Id: I6a97a75a7fb490b03e1f24efd6868edeec98c452

GettingStarted.md

diff --git a/GettingStarted.md b/GettingStarted.md
index b8200f0..da1aa66 100644
--- a/GettingStarted.md
+++ b/GettingStarted.md
@@ -60,9 +60,32 @@
 Now you can build the tree by running:
 
 ```
-m -j$(nproc)
+m
 ```
 
+If you have not modified any packages, and would like to speed your install:
+```
+FETCH_PACKAGES=true m
+```
+This will cause packages to be fetched from Aptitude.
+
+## Artifact caching
+
+There are a few build artifacts that generally don't change, and can be cached to provide a build speedup.
+
+### cache/base.tgz
+- Filesystem tarball for pbuilder. Set FETCH_PBUILDER_DIRECTORY in your environment to the folder containing a copy
+of the file to use an already built version.
+
+### rootdir_ARCH.raw.img
+- Base filesystem tarball made by multistrap. Set ROOTFS_RAW_CACHE_DIRECTORY in your environment to the folder containing a copy
+of the file to use an already built version.
+
+### cache/aiy-board-builder.dar
+- Docker container that the build happens in. Set PREBUILT_DOCKER_ROOT in your environment to the folder containing a copy
+of the file to use an already built version.
+
+
 ## Fastboot
 
 You will need to install Fastboot either via the [Android SDK Manager](https://developer.android.com/studio/#downloads) or through your distro's package manager.
@@ -89,21 +112,6 @@
 flash.sh
 ```
 
-Most of this build relies on cached versions of a Debian bootstrap.
-To rebuild this, remove any copies of debootstrap.tgz from your cache directory
-and run:
-
-```
-DEBOOTSTRAP_FETCH_TARBALL=false m docker-bootstrap
-```
-
-Typically, cached versions of the container images will be fetched.
-To build them from scratch, run the following:
-
-```
-ARM64_BUILDER_FETCH_TARBALL=false DOCKER_FETCH_TARBALL=false m docker-build docker-build-arm64
-```
-
 To build the world for an sdcard, build the `sdcard` target:
 
 ```
@@ -203,10 +211,6 @@
 AndroidThings repository and will likely migrate out as time goes on and bom
 selections change.
 
-#### imx-gpu-viv/
-Contains the Vivante upstream binary drivers for the GPU. These are required to
-get the displays working. Also clonsed from the AndroidThings repository.
-
 #### linux-imx/
 Contains the source code for the Linux kernel.
 
@@ -219,9 +223,6 @@
 Debian package sources used to build the packages installed to make the
 distribution more functional for the Enterprise board.
 
-#### toolchain/
-Contains the prebuilt toolchains to build for an aarch64 target.
-
 #### tools/
 Contains miscellaneous tools to support the image. Bpt and img2simg are forked
 from the Android Things tree, and imx-mkimage from NXP.