Improve the MAUI Nuget readme.md file

### Description

Please refer to [Write a high-quality README for NuGet packages - .NET Blog](https://devblogs.microsoft.com/dotnet/write-a-high-quality-readme-for-nuget-packages/). 

Example of a quality ReadMe file [NuGet Gallery | OpenAI 2.5.0](https://www.nuget.org/packages/OpenAI) package.