Add robots.txt

BUG=skia:
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/851793004
diff --git a/site/robots.txt b/site/robots.txt
new file mode 100644
index 0000000..eb05362
--- /dev/null
+++ b/site/robots.txt
@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: