build directions

doc/BUILDING.md

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+# Build Directions
+
+## EASY METHOD
+
+- Install [nix][nix]
+- Run `nix-shell` in the XeOS folder
+- Run `cargo xbuild` in the XeOS folder
+
+[nix]: https://nixos.org/nix/
+
+## Less easy method
+
+```
+$ rustup install nightly-2020-04-08
+$ rustup default nightly-2020-04-08
+$ cargo install cargo-xbuild
+$ cargo install bootloader
+$ cargo xbuild
+```
+
+Other versions of Rust might work, but the version listed in
+[`rust.nix`][rustnix] is the most accurate.
+
+[rustnix]: https://tulpa.dev/cadey/XeOS/src/branch/master/nix/rust.nix