Add auth file to prove ownership of data.gov.uk

We need to ask google to purge their cached copies of some files. In order for them to trust us we need to prove ownership of data.gov.uk. This adds a file that can be scraped by google to prove we are who we say we are.

public/googlea6393a390aadfbaa.html

@@ -0,0 +1 @@
+google-site-verification: googlea6393a390aadfbaa.html