update readme with test details

Author: sncalvo

README.md

@@ -629,12 +629,18 @@ All config interfaces are exported: `ChartConfig`, `LineConfig`, `BarConfig`, `T
 
 ## Development
 
-Run the test suite:
+Run the Ruby test suite:
 
 ```bash
 ruby -Ilib -Itest -e "Dir['test/**/*_test.rb'].each { |f| require File.expand_path(f) }"
 ```
 
+Run the JavaScript test suite (Vitest + jsdom):
+
+```bash
+npm test
+```
+
 Boot the demo app:
 
 ```bash