Button "Copy to clipboard" instead of "Add to description"

This allows the user to paste the code wherever they need. For example,
a blog post takes its contents from 'properties.content' and not from
the description field.

I also added an explanation for new attachment shortcode.

src/templates/_macros/_node_edit_form.pug

@@ -19,9 +19,9 @@
 
 			| {% if subfield.name.endswith('slug') %}
 			button.fieldlist-action-button.js-append-attachment(
-				type="button")
+				type="button", data-clipboard)
 				i.pi-plus
-				| Append to Description
+				| Copy to clipboard
 			| {% endif %}
 
 			| {% endfor %}

src/templates/layout.pug

@@ -32,6 +32,7 @@ html(lang="en")
 		script(src="{{ url_for('static_pillar', filename='assets/js/vendor/jquery-3.1.0.min.js')}}")
 		script(src="{{ url_for('static_pillar', filename='assets/js/vendor/jquery.typeahead-0.11.1.min.js')}}")
 		script(src="{{ url_for('static_pillar', filename='assets/js/vendor/js.cookie-2.0.3.min.js')}}")
+		script(src="{{ url_for('static_pillar', filename='assets/js/vendor/clipboard.min.js')}}")
 
 		script.
 

src/templates/nodes/edit_embed.pug

@@ -45,6 +45,10 @@
 				i.pi-plus
 				| Add New Attachment
 
+			p.text-muted
+				| Attachments can be included in any MarkDown field by using the #[code {attachment slug}] shortcode.
+				| This shortcode is placed on your copy-paste buffer by clicking "Copy to clipboard".
+
 		| {{ render_field(field, field.name) }}
 
 		| {% elif field.name == 'files' %}