Block search engines via robots.txt (#631)

Prevents instances from being rate limited due to being senselessly
crawled by search engines. Since there is no reason to index Nitter
instances, simply block all robots. Notably, this does *not* affect link
previews (e.g. in various chat software).

public/robots.txt

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