docs(build.zig): document zig build

BUILD.md

@@ -542,3 +542,19 @@ make CMAKE_BUILD_TYPE=Release MACOSX_DEPLOYMENT_TARGET=10.13 DEPS_CMAKE_FLAGS="-
 
 Note that the C++ compiler is explicitly set so that it can be found when the deployment target is set.
 
+## Building with zig
+### Prerequisites
+ - zig 0.15.2
+### Instructions
+ - Build the editor: `zig build`, run it with `./zig-out/bin/nvim`
+ - Complete installation with runtime: `zig build install --prefix ~/.local`
+ - Tests:
+   + `zig build functionaltest` to run all functionaltests
+   + `zig build functionaltest -- test/functional/autocmd/bufenter_spec.lua` to run the tests in one file
+   + `zig build unittest` to run all unittests
+#### Using system dependencies
+  See "Available System Integrations" in `zig build -h` to see available system integrations. Enabling an integration, e.g. `zig build -fsys=utf8proc` will use the system's installation of utf8proc.
+
+`zig build --system deps_dir` will enable all integrations and turn off dependency fetching. This requires you to pre-download the dependencies which don't have a system integration into `deps_dir` (at the time of writing these are ziglua, [`lua_dev_deps`](https://github.com/neovim/deps/blob/master/opt/lua-dev-deps.tar.gz), and the built-in tree-sitter parsers). You have to create subdirectories whose names are the respective package's hash under `deps_dir` and unpack the dependencies inside that directory - ziglua should go under `deps_dir/zlua-0.1.0-hGRpC1dCBQDf-IqqUifYvyr8B9-4FlYXqY8cl7HIetrC` and so on. Hashes should be taken from `build.zig.zon`.
+
+See the `prepare` function of [this `PKGBUILD`](https://git.sr.ht/~chinmay/nvim_build/tree/26364a4cf9b4819f52a3e785fa5a43285fb9cea2/item/PKGBUILD#L90) for an example.