Add build doc (#72)

scripts/fetch-clang-headers.lua

@@ -0,0 +1,18 @@
+import("net.http")
+
+function main()
+    local hash = "fac46469977da9c4e9c6eeaac21103c971190577" -- 2025.01.04
+    local github_url = format("https://raw.githubusercontent.com/llvm/llvm-project/%s/clang/lib/Sema/", hash)
+
+    local headers = {
+        "CoroutineStmtBuilder.h",
+        "TypeLocBuilder.h",
+        "TreeTransform.h",
+    }
+
+    local install_clang_include_dir = path.join(path.directory(path.absolute(os.scriptdir())), "include/clang/Sema")
+
+    for _, header in ipairs(headers) do
+        http.download(github_url .. header, path.join(install_clang_include_dir, header))
+    end
+end