Add some docs (#198)

docs/usage/embed.md

@@ -0,0 +1,11 @@
+# Embed a Gist to your webpage
+
+To embed a Gist to your webpage, you can add a script tag with the URL of your gist followed by `.js` to your HTML page:
+    
+```html
+<script src="http://opengist.url/user/gist-url.js"></script>
+
+<!-- Dark mode: -->
+<script src="http://opengist.url/user/gist-url.js?dark"></script>
+```
+