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.
Clarify the usage of .paths in build.zig.zon. Follow the recommendation
of the comments to explicitly list paths by explicitly listing the paths
in the init project.
