mapster is missing NuGet package README file

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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.

## Get Started with READMEs

### 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.

## We Value Your Feedback
We value your feedback. If you encounter any issues or have suggestions, please reply to this issue.

Thank you for your contribution to the NuGet community.

Start by inspecting the repository's NuGet packaging configuration and existing project documentation to find where package contents and README metadata are defined. Add the package README as described in the linked NuGet guidance, then build the package and verify that the README is included and displayed on NuGet.org.

Written by the indexing model from the issue text.