Add ReadMe.txt for packages

[bijington]

Description of Change

Adds a README.txt to the NuGet Package that provide guidance to consumers on how to Use the toolkit upon installation.

Linked Issues

• Fixes [Proposal] Ship ReadMe.txt file with packages to inform consumers of necessary steps #273

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls/XYZ

Additional information

[bijington]

Also need to update the ReadMe.md file to include the getting started info as per:

#273 (comment)

[bijington]

Also need to apply this to Maui.Markup -> CommunityToolkit/Maui.Markup#27

[bijington]

Final point is to test the behaviour when adding CommunityToolkit.Maui package to see if it opens 2 readme.txt files This works as expected (only showing the package added directly)

[brminnick]

Thanks Shaun!

Just a couple tweaks recommended 👍

[brminnick]

Hey Shaun! I had a thought about the namespace for the UseCommunityToolkit and UseCommunityToolkitCore extension methods.

Let me know what you think!

[brminnick]

Thanks Shaun!!