update_engine: Install payload signing public key

BUG: 23937688
Change-Id: I931a196e207643077185b7bc904744cd42f82c32
diff --git a/update_payload_key/README b/update_payload_key/README
new file mode 100644
index 0000000..cf87148
--- /dev/null
+++ b/update_payload_key/README
@@ -0,0 +1,2 @@
+This directory contains the public half of the payload signing key. This is
+baked into the system image as part of update_engine install.