docs: add systemd hyper links and install steps

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Correct step order, prompt and title (README.md).
 - Correct clone URL (README.md).
+- Add Systemd hyperlink (README.md).
+- Add missing install steps for user and group (README.md).
 
 ## [1.0.0] - 2020-02-06
 ### Added

README.md

@@ -18,6 +18,7 @@ Systemd unit files for use with the Minecraft Server and associated services
 - GNU Linux operating System
 - [Git](https://git-scm.com/)
 - [Wardenctl](https://gitlab.com/adouglas/wardenctl) (optional if one replaces wardenctl commands)
+- [Systemd](https://freedesktop.org/wiki/Software/systemd/)
 
 # Installation
 
@@ -66,9 +67,10 @@ at the CLI.
    minecraft-vanilla.service
    ```
 1. Edit each applicable unit file(s).
-   - Ensure paths are correct (ReadWriteDirectories, WorkingDirectory, ExecStart,
-ExecStop).
-   - Ensure timers property "OnUnitActiveSec" (backup interval) is set as
+   - Ensure paths are correct (ReadWriteDirectories, [WorkingDirectory](https://www.freedesktop.org/software/systemd/man/systemd.exec.html),
+[ExecStart](https://www.freedesktop.org/software/systemd/man/systemd.service.html),
+[ExecStop](https://www.freedesktop.org/software/systemd/man/systemd.service.html)).
+   - Ensure timers property "[OnUnitActiveSec](https://www.freedesktop.org/software/systemd/man/)" (backup interval) is set as
 desired.
    ```console
    $ nano minecraft-backup.service
@@ -81,6 +83,23 @@ desired.
    ```console
    # chown root:root /etc/systemd/system/minecraft-*
    ```
+1. Add user base upon "User" unit property value.
+   ```
+   # useradd -r -s /bin/bash minecraft
+   ```
+   ```
+   -r create a system account.
+   -s user's login shell.
+   ```
+1. Add group based upon "Group" unit property value.
+   - **Note:** This step is only required if group name differs from username.
+   ```
+   # groupadd minecraft
+   # usermod -a minecraft minecraft
+   ```
+   ```
+   -a add the ser to supplementary group.
+   ```
 1. Enable services.
    ```console
    # systemctl enable minecraft-paper.service