docs: bundler auth/hostRules (#5824)

lib/manager/bundler/readme.md

@@ -1 +1,26 @@
 The `bundler` manager is used to extract dependencies from `Gemfile` and `Gemfile.lock` files.
+
+### Authenticating private registry
+
+If you need Bundler to authenticate with a private registry - and it's not the same host as your GitHub/GitLab/etc - then you should do so with `hostRules` and be sure to set the `hostType` value to be "bundler". e.g.
+
+```json
+{
+  "hostRules": [
+    {
+      "hostname": "private-registry.company.com",
+      "hostType": "bundler",
+      "token": "abc123"
+    }
+  ]
+}
+```
+
+Important notes regarding the above:
+
+`hostType=bundler` is essential. If the registry is used for multiple package types then you may need multiple `hostRules`. You cannot leave off `hostType`.
+
+If you don't want to commit raw secrets to your repository, either:
+
+- If self hosting, add the `hostRules` to your bot's configuration file rather than the repository's configuration file, or
+- If using the hosted WhiteSource Renovate app, make use of the [`encrypted`](https://docs.renovatebot.com/configuration-options/#encrypted) capability