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 56f86522a2f8b2a5f48c77fae34028d7031f8b60..c3afb0f694321a29c585221c5b62123c339339e4 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 (required on POSIX, ignored on non-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.
+  - `ar`: 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