Add recommendations on Publishing libraries to Maven Central

[rtyley]

What is being recommended?

The main message is that Scala projects should use gha-scala-library-release-workflow to publish to Maven Central, as we've standardised on it and it does all the right things:

• achieve zero-onboarding for new developers: Any developer who has write access to a repo should be able
  to publish a release of the library, at the click of a button.
• securely handle release credentials - only allow access to release credentials for parts of the release process
  that need them.
• automated version compatibility checking - to avoid binary-incompatibility causing runtime errors.
• reduce per-repo config - adding library-publishing to a repo should add minimal boilerplate.

As Scala is not the only JVM-language that we publish to Maven Central- we have several Android/Kotlin projects! - I've tried to make the document generally applicable for publishing to Maven Central, regardless of JVM-based language.

What's the context?

The prompt to do this now comes from our plan to significantly reduce the number of users with admin access to Sonatype to a minimal number - @davidfurey suggested it would be a good idea to have clear documentation for other developers about who the remaining admins are, in the case of an emergency! So this documentation provides that, along with other guidance about publishing to Maven Central.

The Google Drive documents for those details, linked to from this public markdown doc, are not public, but have constrained access appropriate to their audience.

[davidfurey]

Thanks Roberto!

[davidfurey]

Perhaps this should be linked to from https://github.com/guardian/recommendations/blob/main/README.md ?

[rtyley]

Perhaps this should be linked to from https://github.com/guardian/recommendations/blob/main/README.md ?

Ah good point! I've revised my commit to update the main README.md with a link 👍