mqttnet is missing NuGet package README file

We've noticed that your package on NuGet.org is missing a README file.

## Why READMEs are Important
A README is essential for users to understand your package's purpose and functionality quickly. It's the first thing users see on NuGet.org and helps them decide if your package meets their needs.

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### How to Add a README
If you're new to NuGet READMEs, follow guidance on [how to add one to your package](https://devblogs.microsoft.com/nuget/add-a-readme-to-your-nuget-package/).

### How to write high quality of READMEs
Follow [a blog post](https://devblogs.microsoft.com/nuget/write-a-high-quality-readme-for-nuget-packages/) with README best practices and a template file to help you get started.

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