Chore(docs): Move CONTRIBUTOR.MD to CONTRIBUTING.MD

[IanEdington]

Based on Snyk health dash we should name this file contributING rather than contributOR.

https://snyk.io/advisor/npm-package/mjml-react?utm_medium=referral&utm_source=skypack&utm_campaign=snyk-widget

[github-actions[bot]]

:tada: This PR is included in version 3.1.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[github-actions[bot]]

:tada: This PR is included in version 4.0.0-main-alpha.2 :tada:

The release is available on:

• npm package (@main-alpha dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: