Document .well-known E2EE secure backup setting

docs/e2ee.md

@@ -7,10 +7,36 @@ For private room creation, Element will default to encryption on but give you th
 
 Set the following on your homeserver's
 `/.well-known/matrix/client` config:
+
 ```json
 {
-  "im.vector.riot.e2ee": {
+  "io.element.e2ee": {
     "default": false
   }
 }
 ```
+
+# Secure backup
+
+By default, Element strongly encourages (but does not require) users to set up
+Secure Backup so that cross-signing identity key and message keys can be
+recovered in case of a disaster where you lose access to all active devices.
+
+## Requiring secure backup
+
+To require Secure Backup to be configured before Element can be used, set the
+following on your homeserver's `/.well-known/matrix/client` config:
+
+```json
+{
+  "io.element.e2ee": {
+    "secure_backup_required": true
+  }
+}
+```
+
+# Compatibility
+
+The settings above were first proposed under a `im.vector.riot.e2ee` key, which
+is now deprecated. Element will check for either key, preferring
+`io.element.e2ee` if both exist.