Allow overwriting app.element.io when popping out widgets (#1277)

docs/SUMMARY.md

@@ -10,3 +10,7 @@
 # Distribution
 
 -   [Updates](updates.md)
+
+# Setup
+
+-   [Config](config.md)

docs/config.md

@@ -0,0 +1,15 @@
+# Configuration
+
+All Element Web options documented [here](https://github.com/vector-im/element-web/blob/develop/docs/config.md) can be used as well as the following:
+
+---
+
+The app contains a configuration file specified at build time using [these instructions](https://github.com/vector-im/element-desktop/#config).
+This config can be overwritten by the end using by creating a `config.json` file at the paths described [here](https://github.com/vector-im/element-desktop/#user-specified-configjson).
+
+After changing the config, the app will need to be exited fully (including via the task tray) and re-started.
+
+---
+
+1. `update_base_url`: Specifies the URL of the update server, see [document](https://github.com/vector-im/element-desktop/blob/develop/docs/updates.md).
+2. `web_base_url`: Specifies the Element Web URL when performing actions such as popout widget. Defaults to `https://app.element.io/`.