docs: note that node logs may also be in the system journal — heartwood

See https://radicle.zulipchat.com/#narrow/channel/369873-support/topic/I.20cannot.20access.20logs.20on.20a.20node/near/480509559

Signed-off-by: Lars Wirzenius liw@liw.fi

 After your node starts, it will attempt to connect to peers on the network.
 Note that if you set up your node to be started using **systemd**(1),
 the logs will go to the system journal. These can be viewed with **journalctl**(1):
     $ journalctl -u radicle-node -S today
 To connect to a specific peer, the *connect* sub-command can be used:
 For example, to connect to the *radicle.garden* community node, you can run: