Description

It bugs me that I see this outdated README every time I open the flutter framework package for reading or contributing.

I adjusted the header syntax to match the main README and other READMEs of packages in the framework (e.g. flutter_localizations or flutter_tools).
Additionally, I made sure that the information in the short description now also matches the inclusion of web and desktop efforts.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read [Handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.