Add instructions to Modern theme.

That all may experience the joy of colored skill progress bars.

themes/modern/README.md

@@ -1,5 +1,28 @@
 Theme: Modern
 ===============
-A contemporary technical résumé theme for FluentCV.
+A contemporary technical résumé theme for FluentCV with colored progress bars
+representing skill levels and Font Awesome icons for section headers.
+
+Note: In order to display colored progress bars as in the below image, your resume should include a `level` annotation for each skill:
+
+```json
+{
+  "name": "JavaScript",
+  "level": "master",
+  "skills": [
+    "Node.js", "Angular.js", "jQuery", "Bootstrap", "React.js", "Backbone.js"
+  ]
+},
+{
+  "name": "Database",
+  "level": "intermediate",
+  "skills": [
+    "MySQL", "PostgreSQL", "NoSQL", "ORM", "Hibernate"
+  ]
+}
+```
+
+Valid values for `level` are `master`, `advanced`, `intermediate`, `beginner`,
+or a number between 1 and 10.
 
 ![](modern.png)