luci-py/isolateserver.py: Add archive support when downloading.

appengine/isolate/doc/Design.md

Index: appengine/isolate/doc/Design.md
diff --git a/appengine/isolate/doc/Design.md b/appengine/isolate/doc/Design.md
index b123cd9b0e973af524fb2b5f00e1863d413cd6dd..65c7e69d4fc0f128a9e4e8c439369186f9a5749e 100644
--- a/appengine/isolate/doc/Design.md
+++ b/appengine/isolate/doc/Design.md
@@ -117,6 +117,7 @@ The root is a dictionary with the following keys:
     - `l`: link destination iff a symlink
     - `m`: posix file mode iff on posix
     - `s`: file size iff not a symlink
+    - `t`: type of the file iff not the default of `basic`
   - `includes`: references another `.isolated` file for additional files or to
     provide the command. In practice, this is used to reduce `.isolated` file
     size by moving rarely changed test data files in a separate `.isolated`
@@ -130,6 +131,14 @@ The root is a dictionary with the following keys:
     version shouldn't be able to parse it.
 
 
+##### File types
+
+There are two file types;
+  - `basic`: All normal files, the default type.
+  - `smallar`: An [ar](https://en.wikipedia.org/wiki/Ar_(Unix))
+    archive containing a large number of small files.
+
+
 #### Arbitrary split vs recursive trees
 
 The `.isolated` format supports the `includes` key to split and merge back list