Plugin development on the open source IDEA.

I following the document provided in the website, I did not found a IDEA SDK option in the SDK panel.
How to do it ?
Why provide the SDK as a plugin in the Plugins panel, so the plugin developer can install it derectly.

By the way,
1. Why create a new Forum for the open source IDEA discussion?
2. I hope the open source IDEA  migrate to the modular system( OSGI or others), and make it as a application platform , not only a IDE platform.