ogma-cli: Update contribution guidelines. Refs #161.

[ivanperez-keera]

Update contribution guidelines to indicate that contributions are welcome, and give basic directions, as prescribed in the solution proposed for #161.

[ivanperez-keera]

Change Manager: Verified that:

• Solution is implemented:
  • The code proposed compiles and passes all tests. Details:
    • Build logs:
      • https://github.com/nasa/ogma/actions/runs/11985916839/job/33418401337
      • https://github.com/nasa/ogma/actions/runs/11985916828/job/33418401303
    • Build log archives:
      • logs_31246279641.zip
      • logs_31246279655.zip
  • The solution proposed produces the expected result. Details:
    The change is to the README, so it's been evaluated by manual inspection. I have checked that the instructions invite users to submit, that they are precise, that they address the main concerns, and that they are simple and clear.
• Implementation is documented. Details:
  The change is to the documentation.
• Change history is clear.
• Commit messages are clear.
• Changelogs are updated.
• Examples are updated. Details:
  No updates needed.
• Required version bumps are evaluated. Details:
  Bump not needed. Changes affect README only.