[#453] Add documentation for rollup node service

Problem: Use of rollup node systemd service is undocumented.

Solution:
* Explain how to start the service
* Explain configuration options
* Give an example of configuration

README.md

@@ -31,6 +31,8 @@ tezos-setup-wizard
 Read [the dedicated article](./docs/baking.md) to find out more about the setup,
 the binaries, and the services used.
 
+For setting up experimental transaction rollup node, see [this doc](./docs/tx_rollup.md).
+
 ### Voting on Ubuntu
 
 An interactive voting wizard is provided for Ubuntu. After setting up a baking instance

docs/tx_rollup.md

@@ -0,0 +1,43 @@
+<!--
+   - SPDX-FileCopyrightText: 2022 Oxhead Alpha
+   - SPDX-License-Identifier: LicenseRef-MIT-OA
+   -->
+
+# Setting up transaction rollup node on Ubuntu
+
+`proto` variable can hold every active protocol, currently `013-PtJakart` and `014-PtKathma`.
+
+By default, `tezos-tx-rollup-node-$proto.service` will use:
+* `/var/lib/tezos/.tezos-client` as the `tezos-client` data directory, set in `DATA_DIR`
+* `http://localhost:8732` as the `tezos-node` RPC address, set in `NODE_RPC_ENDPOINT`
+* `127.0.0.1:8472` as the `tezos-tx-rollup-node-$proto` RPC address, set in `ROLLUP_NODE_RPC_ENDPOINT`
+* `observer` as the `tezos-tx-rollup-node-$proto` working mode, set in `ROLLUP_MODE`
+* `rollup` as the rollup alias, set in `ROLLUP_ALIAS`
+
+At first you should originate rollup with the provided alias for some implicit account:
+```
+sudo -u tezos tezos-client originate tx rollup <ROLLUP_ALIAS> from <IMPLICIT_ACCOUNT_ALIAS>
+```
+
+After that, with the active `tezos-node` service available with the provided uri, run the following command
+```
+systemctl start tezos-tx-rollup-node-$proto.service
+```
+
+In order to change the defaults, open `/etc/default/tezos-tx-rollup-node-$proto` and modify the variables:
+
+```
+DATA_DIR="/var/lib/tezos/.tezos-client"
+NODE_RPC_ENDPOINT="http://localhost:8732"
+ROLLUP_NODE_RPC_ENDPOINT="127.0.0.1:8472"
+ROLLUP_MODE="operator"
+ROLLUP_ALIAS="custom-rollup"
+```
+
+Save and close the editor, restart the service:
+
+```
+sudo systemctl restart tezos-tx-rollup-node-$proto.service
+```
+
+For futher details, see [the documentation](http://tezos.gitlab.io/active/transaction_rollups.html).