add systemd logo to README.md

The logo in the readme is hosted by github as the systemd group avatar.

Signed-off-by: Christian Rebischke <chris@nullday.de>

README.md

@@ -1,5 +1,7 @@
 # systemd - System and Service Manager
 
+![systemd logo](https://avatars0.githubusercontent.com/u/1918868?s=200&v=4)
+
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