BUILDING: add missing setup information

[tjkirch]

Description of changes:

While setting up a new machine, I found that I had to do a couple things that weren't yet in the guide.

• openssl is needed to create key/role that we put into images
• lz4 is used to compress/decompress images during build
• perl-FindBin and perl-lib are used when building openssl-sys for cargo-deny (these were already on my Ubuntu machine from the perl-base and perl-modules packages, without my having to install them)

Testing done:

Set up a new build machine and confirmed cargo make works after these additions.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[tjkirch]

^ This push adds the error message that a user would see, so they know if they have to disable the unified cgroups hierarchy.