Add packages/gems/js/README.md

Author: kateinoigakukun

packages/gems/js/README.md

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+# js
+
+[![Gem Version](https://badge.fury.io/rb/js.svg)](https://badge.fury.io/rb/js)
+
+JavaScript bindings for ruby.wasm. This gem provides a way to use JavaScript functionalities from Ruby through WebAssembly.
+
+## Versioning and Compatibility
+
+This gem follows the versioning of ruby.wasm. The version of this gem is the same as the version of ruby.wasm it is compatible with.