Add tip about specifying the full path when creating new wiki pages

app/assets/stylesheets/pages/wiki.scss

+.new-wiki-page {
+  .new-wiki-page-slug-tip {
+    display: inline-block;
+    max-width: 100%;
+    margin-top: 5px;
+  }
+}
+
 .title .edit-wiki-header {
   width: 780px;
   margin-left: auto;

app/views/projects/wikis/_new.html.haml

@@ -13,5 +13,9 @@
               = label_tag :new_wiki_path do
                 %span Page slug
               = text_field_tag :new_wiki_path, nil, placeholder: 'how-to-setup', class: 'form-control', required: true, :'data-wikis-path' => namespace_project_wikis_path(@project.namespace, @project), autofocus: true
+              %span.new-wiki-page-slug-tip
+                %i.fa.fa-lightbulb-o
+                 Tip: You can specify the full path for the new file.
+                We will automatically create any missing directories.
             .form-actions
               = button_tag 'Create Page', class: 'build-new-wiki btn btn-create'