[google_fonts] Improve CONTRIBUTING and generator README

packages/google_fonts/CONTRIBUTING.md

@@ -1,8 +1,2 @@
-## Updating the fonts
-
 If you notice fonts that are on [fonts.google.com](https://fonts.google.com) that do not appear in
-this package, it means that the generator needs to be run. The generator will
-check [fonts.google.com](https://fonts.google.com) for any new fonts, manually test each URL, and
-regenerate the dart code.
-
-To run it manually, run `dart generator/generator.dart`.
+this package, it means that the [generator](./generator/README.md) needs to be run to update the package.

packages/google_fonts/generator/README.md

@@ -1,7 +1,9 @@
-Generates the `GoogleFonts` class, list of supported families.
+The generator will check [fonts.google.com](https://fonts.google.com) for new fonts, validate each URL, and
+regenerate most Dart code (e.g. `GoogleFonts` class), and [families_supported](./families_supported).
 
-1. Navigate to the root directory of this project.
+Note: Googlers only, pending b/280786655, there is an additional prerequisite step required. Contact the Google Fonts team from the linked issue.
+
+1. Navigate to the root directory of this project ([packages/google_fonts](..)).
 2. `dart generator/generator.dart`
 
-See generator/families\_diff for a summary of changes, which is useful when
-generating CHANGELOG.md entries.
+After generation, see the `families_diff` file for a summary of changes, which can be useful for writing `CHANGELOG.md` entries.