Add: usercss documentation link in the editor

_locales/en/messages.json

@@ -329,6 +329,10 @@
     "message": "Support",
     "description": "Label for the external link to style's support site"
   },
+  "externalUsercssDocument": {
+    "message": "Documentation for Usercss",
+    "description": "Label for the external link to usercss documentation"
+  },
   "filteredStyles": {
     "message": "$numShown$ shown of $numTotal$ total",
     "description": "TL note - make this message short",

edit/edit.css

@@ -134,6 +134,10 @@ h2 .svg-icon, label .svg-icon {
 #options .option > * {
   padding-right: 0.25rem;
 }
+/* footer */
+#footer {
+  margin-top: 1em;
+}
 /************ content ***********/
 #sections > div {
   margin: 0.7rem;

edit/source-editor.js

@@ -17,6 +17,11 @@ function createSourceEditor(style) {
     ]})
   );
 
+  $('#header').appendChild($element({
+    id: 'footer',
+    appendChild: makeLink('https://github.com/openstyles/stylus/wiki/Usercss', t('externalUsercssDocument'))
+  }));
+
   setupOptionsExpand();
 
   // dirty reporter