Update README with new instructions on how to run the gentest script (#1502)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1502

We should change this if we are moving away from using Ruby

Reviewed By: yungsters, NickGerleman

Differential Revision: D51957351

fbshipit-source-id: fddb0b3348db2b93c661c919a61150c81906a61a

package.json

@@ -5,7 +5,8 @@
   "scripts": {
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
-    "tsc": "yarn workspaces run tsc"
+    "tsc": "yarn workspaces run tsc",
+    "gentest": "node --loader=babel-register-esm gentest/gentest-driver.ts"
   },
   "workspaces": [
     "javascript",
@@ -15,6 +16,7 @@
   "devDependencies": {
     "@typescript-eslint/eslint-plugin": "^5.30.5",
     "@typescript-eslint/parser": "^5.30.5",
+    "babel-register-esm": "^1.2.5",
     "eslint": "^8.30.0",
     "eslint-config-prettier": "^8.5.0",
     "eslint-plugin-jest": "^27.1.7",