Correct IsWarnEnabled property in README

[winscripter]

Hi, I noticed a small discrepancy in the README example.

1. The IMp4Logger interface doesn't have WarnEnabled - it's IsWarnEnabled instead.
2. Properties like IsWarnEnabled or IsErrorEnabled are actually get-only. They don't have a set accessor. I can make a new PR for them to support the set accessor if you'd like

This PR fixes the former.

I would also like to make new PRs for performance and code quality improvements, if you’d be open to it.

[jimm98y]

I haven't got time to test it yet. I'll have to update the APIs in my other projects using it as well. Yes, the accessor should be added - can you please add it to this PR as well?

[winscripter]

@jimm98y Done, set accessor has been added.

[winscripter]

I would also like to submit PRs to improve performance as well as code quality to use latest C# language features, given that LangVersion is set to latest. Would you like me to work on refactoring and performance improvements on a new PR?

[jimm98y]

Latest C# features are certainly nice, but in my opinion they don't always improve code quality and they make the code less portable and sometimes harder to understand. If it would provide a meaningful performance bump then sure, I'd say go for it and we can discuss it over some performance benchmarks. Bear in mind this project is still targeting netstandard2.0 so not all latest features will work without polyfills.