[#10] Update gem usage in README file

[unijewski]

When we use the gem with simple_form then we get a warning regarding heartcombo/simple_form#997 described in #10 However, the recommended solution like:

def input(wrapper_options)
    @builder.text_field(attribute_name, merge_wrapper_options(input_html_options, wrapper_options))
end

Doesn't work properly in our case because wrapper_options hash looks like:

{:class=>"form-control"}

And input_html_options hash after the merge looks like:

{:preview=>true, :rows=>20, :class=>"wmd-input", :placeholder=>nil, :id=>"wmd-input-body"}

When we merge these two hashes using merge_wrapper_options helper as a result we get

{:preview=>true, :rows=>20, :class=>nil, :placeholder=>nil, :id=>"wmd-input-body"}

As you can see class key is a nil but we need both recent values, so:

  def merge_options(html_opts, wrapper_opts)
    html_opts.merge(wrapper_opts) { |_key, first, second| first + ' ' + second }
  end

seems to work pretty well.