Add a UI guide note about number_with_delimiter

[ci skip]

doc/development/ui_guide.md

@@ -57,3 +57,9 @@ Do not use both green and blue button in one form.
 * For all other cases use default white button.
 * Text button should have only first word capitalized. So should be "Create issue" instead of "Create Issue"
 
+## Counts
+
+* Always use the [`number_with_delimiter`][number_with_delimiter] helper to
+  display counts in the UI.
+
+[number_with_delimiter]: http://api.rubyonrails.org/classes/ActionView/Helpers/NumberHelper.html#method-i-number_with_delimiter