docs: Update contributing instructions.

[jgonzalez-at-wiris]

Description

Just added some info in contributing instructions.

Type of Change

Please delete options that are not relevant.

• Feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Chore (non-breaking change that doesn't add any functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (changes to documentation only)
• Refactoring (non-breaking change that improves the code structure)

Checklist

• My code follows the style guidelines of this project ( Run yarn lint to check)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• New and existing unit tests pass locally with my changes

How should be tested? (Manual or Automated Tests)

Just read the changes.