Document release process

[toniedzwiedz]

The process of placing plugins in the SonarQube Marketplace is changing https://community.sonarsource.com/t/no-need-for-new-threads-for-plugin-updates/70351

https://community.sonarsource.com/t/deploying-to-the-marketplace/35236

This is a good moment to document the necessary steps in a RELEASING.md document.