Encourage contributions in README and CONTRIBUTING

[peanutfun]

Changes proposed in this PR:

• Be more encouraging about contributing to CLIMADA in README.md and CONTRIBUTING.md.
• Add a list and explanation of possible contributions.

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• [ ] Documentation updated
• [ ] Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
• No new linter issues

[chahank]

Happy, thanks!

[chahank]

The changelog has now a conflict :D But otherwise feel free to merge!

[peanutfun]

Done. Builds have been passing, no Python files were changed. Merging now.