diff --git a/README.md b/README.md
index b8ac6e5..37eb2a7 100644
--- a/README.md
+++ b/README.md
@@ -6,3 +6,19 @@
 #Dependencies
 
 `quilt kpartx realpath qemu-user-static debootstrap zerofree pxz`
+
+#Config
+
+Environment and other variables may be provided in a file named `config` in
+your current working directory when you run `build.sh`.  At the moment, the
+only thing you must configure is the name of the image to create.  Something
+like this is used for Raspbian:
+
+```bash
+IMG_NAME='Raspbian'
+```
+
+Obviously if you are making changes to the pi-gen stages you should probably
+use a different `IMG_NAME` to avoid confusion.
+
+You can also define `APT_PROXY` here if you need to.