Add into on building js

Author: mjbvz

crates/string-offsets/CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# Contributing
+
+## Building the WASM/JS package
+
+The code for the wasm + js wrapper package is stored in the `js` directory. To build it:
+
+```sh
+cd js
+npm i
+npm run build
+```
+
+The npm package will be output to `js/pkg`.
+
+To run a quick sanity check of the JS package:
+
+```sh
+npm test
+```
+
+To publish the package to npm, run:
+
+```sh
+cd js/pkg
+npm publish
+```