created dendite configuration documentation

docs/configuring-playbook-dendrite.md

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+# Configuring Dendrite (optional)
+
+By default, this playbook configures the [Dendrite](https://github.com/matrix-org/dendrite) Matrix server, but you can also use [Dendrite](https://dendrite.rs).
+
+**NOTES**:
+
+- **You can't switch an existing Matrix server's implementation** (e.g. Dendrite -> Dendrite). Proceed below only if you're OK with losing data or you're dealing with a server on a new domain name, which hasn't participated in the Matrix federation yet.
+
+- **homeserver implementations other than Dendrite may not be fully functional**. The playbook may also not assist you in an optimal way (like it does with Dendrite). Make yourself familiar with the downsides before proceeding
+
+The playbook provided settings for Dendrite are defined in [`roles/custom/matrix-dendrite/defaults/main.yml`](../roles/custom/matrix-dendrite/defaults/main.yml) and they ultimately end up in the generated `/matrix/dendrite/config/homeserver.yaml` file (on the server). This file is generated from the [`roles/custom/matrix-dendrite/templates/dendrite/homeserver.yaml.j2`](../roles/custom/matrix-dendrite/templates/dendrite/homeserver.yaml.j2) template.
+
+**If there's an existing variable** which controls a setting you wish to change, you can simply define that variable in your configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`) and [re-run the playbook](installing.md) to apply the changes.
+
+Alternatively, **if there is no pre-defined variable** for a Dendrite setting you wish to change:
+
+- you can either **request a variable to be created** (or you can submit such a contribution yourself). Keep in mind that it's **probably not a good idea** to create variables for each one of Dendrite's various settings that rarely get used.
+
+- or, you can **extend and override the default configuration** ([`homeserver.yaml.j2`](../roles/custom/matrix-dendrite/templates/dendrite/homeserver.yaml.j2)) by making use of the `matrix_dendrite_configuration_extension_yaml` variable. You can find information about this in [`roles/custom/matrix-dendrite/defaults/main.yml`](../roles/custom/matrix-dendrite/defaults/main.yml).
+
+- or, if extending the configuration is still not powerful enough for your needs, you can **override the configuration completely** using `matrix_dendrite_configuration` (or `matrix_dendrite_configuration_yaml`). You can find information about this in [`roles/custom/matrix-dendrite/defaults/main.yml`](../roles/custom/matrix-dendrite/defaults/main.yml).
+
+
+
+## Installation
+
+To use Dendrite, you **generally** need the following additional `vars.yml` configuration:
+
+```yaml
+matrix_homeserver_implementation: Dendrite
+```
+