Added docs for Deprecator in CONTRIBUTING.md

[keshavbiswa]

Motivation / Background

This Pull Request has been created because we need to provide docs for Deprecator in CONTRIBUTING.md

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug, refactor something, or add a feature.
• Tests and Rubocop are passing before submitting your proposed changes.

If you're proposing a new generator or locale:

• Double-check the existing generators documentation to make sure the new generator you want to add doesn't already exist.
• You've reviewed and followed the Contributing guidelines.

[stefannibrasil]

HI @keshavbiswa thank you so much! This was in my TODO list and it's nice that you did it already. Just left two suggestions. Thanks!

[stefannibrasil]

thank you!