docs: add asdf and GitHub Actions installations

[ronnnnn]

What does this change?

Add asdf and GitHub Actions installations on README.

• FlutterGen/asdf-fluttergen
• FlutterGen/setup-fluttergen

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

Checklist:

Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• [x] Make sure to open a GitHub issue as a bug/feature request before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
  • [x] Ensure the tests (melos run test)
  • [x] Ensure the analyzer and formatter pass (melos run format to automatically apply formatting)
• [x] Appropriate docs were updated (if necessary)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.52%. Comparing base (a1c15a8) to head (d0aa8ee). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #560 +/- ## ======================================= Coverage 97.52% 97.52% ======================================= Files 24 24 Lines 848 848 ======================================= Hits 827 827 Misses 21 21 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[wasabeef]

@ronnnnn

Thanks!

[ronnnnn]

🙏