Update README common PR steps to be more beginner friendly

Author: heejongahn

README.md

@@ -27,8 +27,9 @@ The common steps to send a pull request are:
 
 0. Open or refer to an issue in the [TypeScript repo](https://github.com/Microsoft/TypeScript).
 1. Add missing elements to `inputfiles/addedTypes.json`, overriding elements to `inputfiles/overridingTypes.json`, or elements to remove to `inputfiles/removedTypes.json`.
-2. Run the script locally to obtain new `dom.generated.d.ts` and `webworker.generated.d.ts`.
-3. Update the files in the `baselines` folder using the newly generated files.
+2. Run the build script locally to obtain new `dom.generated.d.ts` and `webworker.generated.d.ts`.
+3. Update the files in the `baselines` folder using the newly generated files
+   under `generated` folder (`cp ./generated/* ./baseline/`).
 
 ### When should a DOM API be included here?