Add readme

README.md

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+# Zig LDtk
+
+This is a single file library for parsing LDtk JSON files.
+
+## Using
+
+Copy `LDtk.zig` into your project and use like so:
+
+```zig
+const LDtk = @import("./LDtk.zig");
+
+// ldtk_file should be a `[]const u8` of the file contents
+const ldtk = LDtk.parse(allocator, ldtk_file);
+defer ldtk.deinit();
+
+// Now you can access your LDtk data through the `ldtk` struct
+for (ldtk.levels) {
+    // ...
+}
+```
+Alternatively, add this repository as a git submodule.