Delete everything from the shareables/ directory to save space on dac.

All of the training ZIPs are now located in development/samples/training/zips/
Added a readme file to explain this change toward using cloud storage instead.

Change-Id: I1f56ac35b88506d0ba0eaa54861e4a2a7d4c566a

docs/html/shareables/README

@@ -0,0 +1,8 @@
+DO NOT PUT ANY FILES IN THIS DIRECTORY
+
+All URLS pointing to this directory redirect to a corresponding location
+at http://commondatastorage.googleapis.com/androiddevelopers/shareables/
+
+This directory was originally created for downloadable items such as ZIP files,
+but we've moved them all to Google Cloud Storage to reduce the size
+of the Android Developers site.