Update docs/configuring-playbook-bridge-beeper-linkedin.md: re-add the section for instruction about appservice double puppeting

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-bridge-beeper-linkedin.md

@@ -4,6 +4,16 @@ The playbook can install and configure [beeper-linkedin](https://github.com/beep
 
 See the project's [documentation](https://github.com/beeper/linkedin/blob/master/README.md) to learn what it does and why it might be useful to you.
 
+## Prerequisite
+
+### Enable Appservice Double Puppet or Shared Secret Auth (optional)
+
+If you want to set up [Double Puppeting](https://docs.mau.fi/bridges/general/double-puppeting.html) (hint: you most likely do) for this bridge automatically, you need to have enabled [Appservice Double Puppet](configuring-playbook-appservice-double-puppet.md) or [Shared Secret Auth](configuring-playbook-shared-secret-auth.md) service for this playbook.
+
+See [this section](configuring-playbook-bridge-mautrix-bridges.md#set-up-double-puppeting-optional) on the [common guide for configuring mautrix bridges](configuring-playbook-bridge-mautrix-bridges.md) for details about setting up Double Puppeting.
+
+**Note**: double puppeting with the Shared Secret Auth works at the time of writing, but is deprecated and will stop working in the future.
+
 ## Adjusting the playbook configuration
 
 To enable the bridge, add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file: