Add an empty robots.txt, mostly just to reduce errors in HTTP log.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@116658 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/www/robots.txt b/www/robots.txt
new file mode 100644
index 0000000..9e60f97
--- /dev/null
+++ b/www/robots.txt
@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: