Add design support library docs

BUG: 20524332
Change-Id: I471ab17a2a5ccc43f4ef0fa04b7c2e9077d56361
diff --git a/core/pathmap.mk b/core/pathmap.mk
index 2532707..137b5d5 100644
--- a/core/pathmap.mk
+++ b/core/pathmap.mk
@@ -114,7 +114,8 @@
         v7/recyclerview \
         v8/renderscript \
         v13 \
-        v17/leanback
+        v17/leanback \
+        design
 
 #
 # A list of all source roots under frameworks/multidex.