improve the GitHub "Community Standards" for this repository

[sheerlox]

https://github.com/kelektiv/node-cron/community

• CODE_OF_CONDUCT.md
• CONTRIBUTING.md
• update issue templates

[ncb000gt]

🎉 This issue has been resolved in version 3.0.0-beta.9 🎉

The release is available on:

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

Your semantic-release bot 📦🚀

[ncb000gt]

🎉 This issue has been resolved in version 3.0.0 🎉

The release is available on:

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

Your semantic-release bot 📦🚀