Creating documentation for testing

[khanniie]

Testing.md provides documentation for how to write testing files.

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest develop branch. (If I was asked to make more changes, I have made sure to rebase onto develop then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

[release-com]

Release Environments

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• p5.js-web-editor
  • app-tedd56c-p5-js-web-editor.releaseapp.io

🔧Environment Status : https://app.releasehub.com/public/Processing%20Foundation/env-c838388b68

[catarak]

Just made some minor copy editing changes :) This is so great!!