Add Breaking Change notes to migration guide

docs/v6-migration.md

@@ -27,6 +27,11 @@ You can grab the new `config.template.js` and simply go through and migrate your
 
 :::
 
+## Breaking Changes
+- qBittorrent: If you use `qbit_manage` to enable AutoTMM, you will need to specify `legacyLinking: false` in your config.
+- API auth: `apiAuth` option was removed and the optional `apiKey` option was added. If unspecified, cross-seed will use an autogenerated API key. Use the API key when you make calls to the `/api/webhook` endpoint and the `/api/announce` endpoint.
+- Minimum required Node version: v20
+
 ### Stricter `config.js` Validation
 
 One of the biggest changes made in v6 is better config validation and error messaging. Each option in the config file is validated for formatting and proper syntax. If either is incorrect, a detailed error message will tell you where and how to fix each option.