Stable OS release with recovery partition

docs/config.md

@@ -31,6 +31,13 @@ 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.
 
+ * `TEMP_REPO` (Default: unset)
+
+   An additional temporary apt repo to be used during the build process. This
+   could be useful if you require pre-release software to be included in the
+   image. The variable should contain sources in [one-line-style format](https://manpages.debian.org/stable/apt/sources.list.5.en.html#ONE-LINE-STYLE_FORMAT).
+   "RELEASE" will be replaced with the RELEASE variable.
+
  * `BASE_DIR`  (Default: location of `build.sh`)
 
    **CAUTION**: Currently, changing this value will probably break build.sh