docs: add downloads directory with initial set of current files.

This directory is not included in the final docs output, but is
merely here as a "master" source for the downloadable bits that we
upload and serve from elsewhere, such as from Google Cloud Storage.

Change-Id: I181438f9ec4513cf2e49455803bbe434c8f29d36

docs/downloads/README

@@ -0,0 +1,9 @@
+Files in this directory are not hosted on developer.android.com.
+
+This directory serves as a "master" repository for various
+downloadable files. This provides us consistent version control
+for downloadables that are associated with documentation. 
+
+Once saved here, the files must be uploaded to a separate
+hosting service where they're accessible by users, such as
+Google Cloud Storage.