Create robots.txt

This GitHub pages site seems to serve from the root of the repo. 
I'm not sure if GitHub pages will serve this file correctly,
but it's worth a shot. Our goal here is to prevent crawlers from
indexing non-prod docs sites.

See Raku/doc-website#384

Author: dontlaugh

robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /