README: add link to OS-specific build instructions

[ronso0]

Adds a quick link to https://github.com/mixxxdj/mixxx/wiki#compile-mixxx-from-source-code with detailed build instructions which are currently hard to find, unless you find your way via Documentation -> Wiki -> scroll down to Developer Documentation -> Compile Mixxx From Source Code

[ronso0]

Ah, IIRC it's also been discussed to add a BUILDING.md or COMPILING.md

edit here is BUILDING.md from #4432

[acolombier]

How would you feel about moving the content of this wiki (the Compile Mixxx From Source Code only, and the page it references) into markdown file(s) under the docs folder? Something like:

docs/building.md (or just BUILDING.md at the root like in #4432)
docs/linux.md
docs/windows.md
docs/macos.md
docs/packaging.md

The rationale for it would be:

• Wiki is RO
• Content is easier to find (e.g grep or ctrl+f on the repo)
• Vendor agnostic - knowledge lives in in Git vs GH Wiki
• Contribution (related to the first point, but implying that it gets easier for a contributor to suggest improvement and request somebody to update these page inline with a change they would submit on our tools scripts)

[ronso0]

Sure, makes sense!

But let's merge this quick fix first.

[ronso0]

I moved your suggestion to #14786