Add skeletal docs for client configuration

docs/config.md

+Configuring the sidebar
+=======================
+
+The Hypothesis sidebar can be configured by providing a settings object in the
+body of the hosting page:
+
+```html
+<script type="application/json" class="js-hypothesis-config">
+  {
+    "openSidebar": true
+  }
+</script>
+<script async src="https://hypothes.is/embed.js"></script>
+```
+
+**N.B.** The body of the `.js-hypothesis-config` tag must be [valid
+JSON](http://jsonlint.com/) -- invalid JSON will cause the entire config object
+to be ignored.
+
+Config keys
+-----------
+
+### `openLoginForm`
+
+_Boolean_. Controls whether the login panel is automatically opened on startup,
+as if the user had clicked "Log in" themselves.
+
+### `openSidebar`
+
+_Boolean_. Controls whether the sidebar opens automatically on startup.