Add license information to docs/configuring-playbook-synapse.md

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

docs/configuring-playbook-synapse.md

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 # Configuring Synapse (optional)
 
 By default, this playbook configures the [Synapse](https://github.com/element-hq/synapse) Matrix server, so that it works for the general case. If that's okay, you can skip this document.