Missing information of localized build for Typescript users.

[ferrywlto]

In our project we are using Typescript with Vue and need to support Traditional Chinese language (zh-TW) locale. Since Typescript requires type checking to work. Dynamically create and append script tag to load the localized version is not an option. Therefore we need to pull the repo and build by our own. Following the README.md doesn't work for us, with firebaseui__zh_tw.js and npm__zh_tw.js, the UI were still in English (en), when inspected the element was still look like this:

<div id="firebase-auth-container" lang="en">

After a few hours of trial and error we find that esm.js was the file we actually needed to be localized. However the README.md didn't mention we can instead build the localized version of esm.js via yarn build build-esm-zh-tw as well. I think this information will help both the new user of firebaseui and TypeScript a lot.

Would the repo owner mind I do a PR to update the README.md with steps on how I successfully build, import and use the localized version?

Below is my successful screenshot:

[bojeil-google]

Thanks, @ferrywlto. Feel free to send a PR to update the README file with the additional instructions.

[brycehuang30]

I follow the removed instruction
It works! Thanks a lot @ferrywlto !