Add mxisd Identity Server support

docs/configuring-playbook-mxisd.md

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+# Adjusting mxisd Identity Server configuration (optional)
+
+By default, this playbook configures an [mxisd](https://github.com/kamax-io/mxisd) Identity Server for you.
+
+This server is private by default, potentially at the expense of user discoverability.
+
+
+## Matrix.org lookup forwarding
+
+To ensure maximum discovery, you can make your identity server also forward lookups to the central matrix.org Identity server (at the cost of potentially leaking all your contacts information).
+
+Enabling this is discouraged and you'd better [learn more](https://github.com/kamax-io/mxisd/blob/master/docs/features/identity.md#lookups) before proceeding.
+
+Enabling matrix.org forwarding can happen with the following configuration:
+
+```yaml
+matrix_mxisd_matrixorg_forwarding_enabled: true
+```
+
+
+## Additional features
+
+What this playbook configures for your is some bare minimum Identity Server functionality, so that you won't need to rely on external 3rd party services.
+
+Still, mxisd can do much more.
+You can refer to the [mxisd website](https://github.com/kamax-io/mxisd) for more details.
+
+You can override the `matrix_mxisd_template_config` variable and use your own custom configuration template.
+
+
+## Troubleshooting
+
+If email address validation emails sent by mxisd are not reaching you, you should look into [Adjusting email-sending settings](configuring-playbook-email.md).