Update docs: jq v1.5 required for makefile build

[rpavlik]

It's being used to generate compile_commands.json to be able to run the various clang tools on the source code base. In this way it's actually technically not required to build, but:

• it is currently in the default target
• it's probably good to ensure that pull requests don't break this functionality (so we should leave it in a pull-request-required makefile target)