update readme

Author: zhengbli

README.md

@@ -10,7 +10,13 @@ To build the tool, simply clone the repo, and run `build.cmd` in command line or
 - `JS.fs`: handles the emitting of the `domWeb.js` and `domWindows.js`
 
 ## Contribution Guide
-The `dom.generated.d.ts` and `webworker.generated.d.ts` files that are used in TypeScript repo `master` branch are used as baselines. For each pull request, we will try run the script and compare the generated files with the baseline files. So to make the tests passed, please update the baseline and the code at the same pull request. 
+The `dom.generated.d.ts` and `webworker.generated.d.ts` files that are used in TypeScript repo `master` branch are used as baselines. 
+For each pull request, we will try run the script and compare the generated files with the baseline files. 
+So to make the tests passed, please update the baseline and the code at the same pull request. 
+
+For common changes, it is sufficient to change the json files. 
+There are three json files that are used to alter the file generation: `addedTypes.json`, `overridingTypes.json` and `removedTypes.json`.
+The format of each file can be inferred from their existing content. 
 
 ## Input Files:
 - `browser.webidl.xml`: the XML spec file generated by Microsoft Edge (due to the different updating schedules between Edge and TypeScript, this may **not** be the most up-to-date version of the spec.);