add OpenADK manual.

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diff --git a/docs/download-location.txt b/docs/download-location.txt
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+// -*- mode:doc; -*-
+// vim: set syntax=asciidoc:
+
+Location of downloaded packages
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The various tarballs that are downloaded by OpenADK are all stored
+in +ADK_DL_DIR+, which by default is the +dl+ directory. If you want
+to keep a complete version of OpenaDK which is known to be working
+with the associated tarballs, you can make a copy of this directory.
+This will allow you to regenerate the toolchain and the target
+filesystem with exactly the same versions.
+
+If you maintain several OpenADK trees, it might be better to have a
+shared download location. This can be achieved by pointing the
++DL_DIR+ environment variable to a directory.  If this is
+set, then the value of +ADK_DL_DIR+ in the OpenADK configuration is
+overridden. The following line should be added to +<~/.bashrc>+.
+
+-----------------
+ $ export DL_DIR=<shared download location>
+-----------------
+
+The download location can also be set in the +.config+ file, with the
++ADK_DL_DIR+ option. Unlike most options in the .config file, this value
+is overridden by the +DL_DIR+ environment variable.