Introduce instructions for measuring test coverage

[kfcampbell]

This small PR adds a README update with instructions for measuring test coverage.

It also includes a go mod tidy update required for the module to build since the downgrade to Kiota v1.14.0.

[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[what-the-diff]

PR Summary

• Documentation Improvement
  A new section has been added to the main README file. This section provides instructions on building and testing our Software Development Kit (SDK). This will help users understand how they can test their developments more effectively.

• Module Addition
  A new module 'golang.org/x/sync v0.7.0' has been included in the project structure. This update in the 'go.mod' file ensures that all developers have access to the same set of functionalities, which aids in consistent development across the team.

• Sync of Dependencies
  The addition of 'golang.org/x/sync v0.7.0' module in the 'go.sum' file ensures that there is a record of the exact version of the module used. This makes sure that every time our project is built, it will use the same version of the module, hence avoiding any inconsistencies due to differences in module versions.