Add instructions for build process to README.

Author: channing-google

README.md

@@ -670,6 +670,15 @@ npm run build
 
 This will create output files in the `dist/` folder.
 
+To build a localized JavaScript binary, run:
+```bash
+npm run build build-js-{LANGUAGE_CODE}
+```
+where `{LANGUAGE_CODE}` is replaced by the
+[code of the language you want](LANGUAGES.md). For example, the French binary
+can be built with `npm run build build-js-fr`. This will create a binary
+`firebaseui__fr.js` in the `dist/` folder.
+
 ### Running the demo app
 
 To run the demo app, you must have a Firebase project set up on the

gulpfile.js

@@ -208,6 +208,7 @@ function makeDefaultFile(fileName) {
 }
 
 // Bundles the FirebaseUI JS with its dependencies for all locales.
+// Generates the gulp tasks build-js-de, build-js-fr, etc.
 const buildJsTasks = repeatTaskForAllLocales('build-js-$',
     ['build-firebaseui-js-$'], (locale) => {
       const localeForFileName = getLocaleForFileName(locale);