Add the ImageCropper document link to readme.md

[HHChaos]

Issue: #2811

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

Readme is missing the document link for ImageCropper.

What is the new behavior?

Add the ImageCropper document link to readme.md.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

[windowstoolkitbot]

This PR is linked to unclosed issues. Please check if one of these issues should be closed: #2811