Add swift-log-systemd.git to the list of available backends (#345) (#345

)

systemd-based log integration

### Motivation:

Using native backend for storing log data

### Modifications:

Add swift-log-systemd to the list of available backends

### Result:

Another one option to use the package with

README.md

@@ -87,6 +87,7 @@ You can choose from one of the following backends to consume your logs. If you a
 | [ShipBook/swift-log-**shipbook**](https://github.com/ShipBook/swift-log-shipbook) | a logging backend that sends log entries to [Shipbook](https://www.shipbook.io) - Shipbook gives you the power to remotely gather, search and analyze your user logs and exceptions in the cloud, on a per-user & session basis. |
 | [kasianov-mikhail/scout](https://github.com/kasianov-mikhail/scout) | CloudKit as a log storage |
 | [PADL/AndroidLogging](https://github.com/PADL/AndroidLogging) | a logging backend for the Android in-kernel log buffer |
+| [PADL/swift-log-systemd.git](https://github.com/PADL/swift-log-systemd.git) | a logging backend for the [systemd](https://systemd.io/) journal |
 | Your library? | [Get in touch!](https://forums.swift.org/c/server) |
 
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