Windows: missing dependency install step in README setup flow #72

On a fresh Windows machine, following the README steps leads to failures:

xlings install d2x -y
d2x install d2mcpp
cd d2mcpp
d2x checker → 'xmake' is not recognized
toolchain("gcc"): not found

While .xlings.json declares these dependencies, xlings install in the
project directory does not resolve them (fails with "package not found").

Fix: add explicit install commands for xmake and Mingw-w64 to the README
and usage guide, so Windows users know what to install before running
d2x checker.