Clarify dependency pitfalls when using direct download for binary packages

[ckreibich]

As pointed out by @dopheide-esnet in Slack:

A user experience issue that we ran across: from https://docs.zeek.org/en/master/install.html they clicked on the "6.0.x" link under the Binary Packages: Linux section. That takes you to https://software.opensuse.org/download.html?project=security%3Azeek&package=zeek-6.0 From there, say you click on Ubuntu, it gives you two options, one to install the repo and one to download the binary package directly. That lets you download just the zeek .deb file. Trying to install that obviously doesn't work because you're missing all of the zeek-6.0-core, zeek-6.0-zkg, zeek-6.0-client, etc dependencies.

We should clarify that adding the repo and then installing via the toplevel package is recommended because it sorts out the dependencies.

@0xxon fyi