Fix broken anchor links and improve contribution guidelines

[ghost]

Proposed changes

As suggested by @ghost in issue #182, this PR fixes broken anchor links and improves the contribution guidelines at CONTRIBUTING.md. As a consequence, it makes the project more inviting for the community to contribute.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the CONTRIBUTING doc
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[coveralls]

Coverage remained the same at 48.438% when pulling 8a3cdbe on bok-jin:pr/fix-broken-links-contributing into 1e69891 on adonisjs:develop.