Update Tools readme to include Keyring Social Importer

[jordesign]

also reorganised the content

What does the code do?

[Provide a description of what the code does]

Code Checklist

• [ ] It contains a readme.md in the root directory.
  • [ ] The readme explains how to use the tool.
  • [ ] The readme explains how to develop the tool.
• [ ] This code has a GPL-compatible license.
• [ ] This code follows [plugin guidelines](https://developer.wordpress.org/plugins/wordpress-org/detailed-plugin-guidelines/

  the-guidelines).

• [ ] I am the original author of this code.