Add README to original Bazel docs directory to point developers to the correct
directory to make documentation changes.

Bug: #579

--
MOS_MIGRATED_REVID=128533607
diff --git a/site/BUILD b/site/BUILD
index ba7fba0..52f9da1 100644
--- a/site/BUILD
+++ b/site/BUILD
@@ -24,6 +24,7 @@
             "*.swp",
             "command-line-reference-prefix.html",
             "command-line-reference-suffix.html",
+            "site/README.md",
         ],
     ),
 )