Document testing on newer Firefox (Cross-Origin Request Blocked)

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS/Errors/CORSRequestNotHttp

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
diff --git a/Readme b/Readme
index b148763..886c034 100644
--- a/Readme
+++ b/Readme
@@ -37,6 +37,10 @@
   ./yaml-to-kml.py -f users/[username].yaml -o www/developers.kml
   firefox www/index.html
 
+You might need to temporarily change the privacy.file_unique_origin in
+about:config to allow accessing local resources in page directory.  Disable the
+setting after tests.
+
 If all looks good, you need to push this information back into the kernel.org
 repository.  Once we have collected enough initial data, we will integrate the
 map into the main kernel.org website.