Add instructions about how to support localhost

Summary:
With T764, http://localhost doesn't work anymore. So add instructions
about how to support it by modifying the hosts file.

Test Plan:
- turned on setup mode and the error message did show up
- turned off the setup mode and the error message also showed up

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T764

Differential Revision: https://secure.phabricator.com/D1370

src/infrastructure/setup/PhabricatorSetup.php

@@ -286,7 +286,10 @@ class PhabricatorSetup {
           "You must host Phabricator on a domain that contains a dot ('.'). ".
           "The current domain, '{$host_domain}', does not have a dot, so some ".
           "browsers will not set cookies on it. For instance, ".
-          "'http://example.com/ is OK, but 'http://example/' won't work.");
+          "'http://example.com/ is OK, but 'http://example/' won't work. ".
+          "If you are using localhost, create an entry in the hosts file like ".
+          "'127.0.0.1 example.com', and access the localhost with ".
+          "'http://example.com/'.");
         return;
       }
     }

webroot/index.php

@@ -234,7 +234,10 @@ function phabricator_detect_bad_base_uri() {
       "'phabricator.base-uri' is set to '{$conf}', which is invalid. The URI ".
       "must contain a dot ('.'), like 'http://example.com/', not just ".
       "'http://example/'. Some web browsers will not set cookies on domains ".
-      "with no TLD, and Phabricator requires cookies for login.");
+      "with no TLD, and Phabricator requires cookies for login. ".
+      "If you are using localhost, create an entry in the hosts file like ".
+      "'127.0.0.1 example.com', and access the localhost with ".
+      "'http://example.com/'.");
   }
 }