Mention docker-desktop log spam issue in Linux section of README

[rmunn]

I ran into docker/desktop-linux#102 today, which was filling up my /var/log/syslog file and thus also my root partition. (It was also filling up the systemd journal, but the systemd journal is capped at 4 GB on my system while I don't have syslog capped). We should probably update the README section for Linux devs to include the workaround, which is to create a systemd drop-in file in your user configuration location ($HOME/.config/systemd/user/docker-desktop-service.d), as follows:

1. Run mkdir -p $HOME/.config/systemd/user/docker-desktop-service.d
2. Create a file called $HOME/.config/systemd/user/docker-desktop-service.d/no-log-spam.conf (it can be any filename, as long as it's in that directory and ends in .conf) with the following content:

[Service]
StandardError=null

No urgency to this task, because we don't expect any new Linux devs to join the team in the next few weeks. But it would be nice to give any new Linux devs a heads-up so they don't also experience their hard disk filling up with debug log messages.

[rmunn]

I thought we had installation instructions for Docker Desktop in the README, but it turns out we don't. So there isn't really a good place to put the mention of that bug in the README. Closing.

[rmunn]

Actually, if we create developer instructions as suggested in #1265, then this would definitely belong in DEVELOPER-linux.md. So I'll tentatively reopen this, pending us making a decision on #1265.