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)
• 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:

• 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
  • Ensure the tests (melos run test)
  • Ensure the analyzer and formatter pass (melos run format to automatically apply formatting)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

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

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[wasabeef]

@ronnnnn

Thanks!

[ronnnnn]

🙏