add synapse_auto_compressor

docs/configuring-playbook-synapse-auto-compressor.md

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+# Setting up synapse_auto_compressor
+
+The playbook can install and configure [synapse_auto_compressor](https://github.com/matrix-org/rust-synapse-compress-state/#automated-tool-synapse_auto_compressor) for you.
+
+It's a cli tool that automatically compresses `state_groups` database table in background.
+
+See the project's [documentation](https://github.com/matrix-org/rust-synapse-compress-state/#automated-tool-synapse_auto_compressor) to learn what it does and why it might be useful to you.
+
+
+## Adjusting the playbook configuration
+
+Add the following configuration to your `inventory/host_vars/matrix.DOMAIN/vars.yml` file:
+
+```yaml
+matrix_synapse_auto_compressor_enabled: true
+```
+
+
+## Installing
+
+After configuring the playbook, run the [installation](installing.md) command again:
+
+```
+ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start
+```
+
+
+## Usage
+
+After installation, synapse_auto_compressor will run automatically every day by the defined schedule
+
+## Manually start the tool
+
+For testing your setup it can be helpful to not wait until 00:00. If you want to run the tool immediately, log onto the server
+and run `systemctl start matrix-synapse-auto-compressor`. This will not return until the run is done, so possibly a long time.
+Consider using [tmux](https://en.wikipedia.org/wiki/Tmux) if your SSH connection is unstable.