The prior three-line CONTRIBUTING left newcomers guessing at every
non-trivial step: which Go / Node versions, where xray comes from, why
the panel goes blank when XUI_DEBUG=true is flipped on, how the Vue
multi-page setup is wired, what to do on Windows when go build trips
on the CGo SQLite driver.
Now covers prerequisites, MinGW-w64 install on Windows (niXman builds
or MSYS2), one-shot first-time setup, two frontend dev workflows with
the XUI_DEBUG asset-cache gotcha called out, the architecture and
conventions of the Vue side, a project-layout map, useful env vars,
and the PR checklist.
