login_form.html.erb

<%= bootstrap_form_for(resource, 
                       as: resource_name, 
                       url: session_path(resource_name)) do |f| %>

    <%= f.email_field :email, 
                      autofocus: true, 
                      class: 'form-control', 
                      placeholder: 'email' %>

    <%= f.password_field  :password, 
                          autocomplete: "off", 
                          class: 'form-control',
                          placeholder: 'password' %>


  <% if devise_mapping.rememberable? -%>
     <%= f.check_box :remember_me %>
  <% end -%>

   <%= f.submit "Log in", class: 'form-control login-button' %>
<% end %>