docs: Add instructions on how to download isolated intermediaries.

site/dev/testing/download.md

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+Downloading Isolates
+====================
+
+The intermediate and final build products from running tests are all stored in
+[Isolate](https://github.com/luci/luci-py/blob/master/appengine/isolate/doc/Design.md),
+and can be downloaded to the desktop for inspection and debugging.
+
+First install the client:
+
+     git clone https://github.com/luci/client-py.git
+
+Add the checkout location to your $PATH.
+
+To download the isolated files for a test first visit
+the build status page and find the "isolated output" link:
+
+<img src="Status.png" style="margin-left:30px" width=576 height=271 >
+
+
+Follow that link to find the hash of the isolated outputs:
+
+
+<img src="Isolate.png" style="margin-left:30px" width=451 height=301 >
+
+Then run `isolateserver.py` with --isolated set to that hash:
+
+    $ isolateserver.py \
+      download \
+      --isolate-server=https://isolateserver.appspot.com \
+      --isolated=5b85b7c382ee2a34530e33c7db20a07515ff9481 \
+      --target=./download/
+