Update robots.txt (getgrav#2632)

I have found that Bing/Yahoo/DuckDuckGo, Yandex and Google report crawl errors when using the default robots.txt. Specifically their bots will not crawl the the path '/' or any sub-paths. I agree that the current robots.txt should work and properly implements the specification. However it still does not work.

In my experience explicitly permitting the path '/' by adding the directive Allow: / resolves the issue.

More details can be found in a blog post about the issue here: https://www.dfoley.ie/blog/starting-with-the-indieweb

Author: dsofeir

robots.txt

@@ -10,3 +10,4 @@ Disallow: /user/
 Allow: /user/pages/
 Allow: /user/themes/
 Allow: /user/images/
+Allow: /