adds note to finish_signup

app/views/users/registrations/finish_signup.html.erb

@@ -3,6 +3,10 @@
 <%= form_for current_user, as: :user, url: do_finish_signup_path, html: { role: 'form'} do |f| %>
   <%= render 'shared/errors', resource: current_user %>
 
+  <div class='callout primary'>
+    <%= t("omniauth.finish_signup.username_warning") %>
+  </div>
+
   <% if current_user.errors.include? :username %>
     <%= f.text_field :username, placeholder: t("devise_views.users.registrations.new.username_label") %>
   <% else %>