Formatters module documentation.

[clrudolphi]

🤔 What's changed?

Adding markdown documentation of the Formatters module as a CONTRIBUTING.md style file.

⚡️ What's your motivation?

Provide sufficient documentation that other Reqnroll developers can maintain and extend the Formatters subsystem.

🏷️ What kind of change is this?

• 📖 Documentation (improvements without changing code)

♻️ Anything particular you want feedback on?

• Topic coverage: what topics are missing or too lightly treated?
• Are the diagrams helpful?
• What other types of documentation artifacts would be helpful?

📋 Checklist:

• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "[vNext]" section of the CHANGELOG, linking to this pull request & included my GitHub handle to the release contributors list.

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This text was originally taken from the template of the Cucumber project, then edited by hand. You can modify the template here.

[clrudolphi]

I intend to create a similar set of documentation for the Formatters tests project.