Document maintanance mode and migration recommendation

[szpak]

IMPORTANT. To make releasing to Maven Central even easier, I and Marc Phillip (the author of nexus-publish-plugin) combined forces to create a next generation, unified, 2-in-1 plugin - gradle-nexus-publish-plugin. It is a recommended solution, as our development effort will be put in that new plugin. See my blog post and the official migration guide.

Thank you for over 5 years of releasing with my plugin!