There's been some proliferation of dependency URLs that reference
mutable data such as links to git branches that can change. This has
resulted in broken projects, i.e.
* 9eef9de94c/build.zig.zon
* 4b64353e9c
There's also disagreement about whether it's fine for URL's to point to
git branches, i.e.
https://github.com/Not-Nik/raylib-zig/pull/130
This updates the docs to mention that zig won't be able to use URLs if
their content changes.