Build global plugin for WordPress multisite

I would like to build a simple plugin for WordPress – just one textarea field to take notes about the multisite installation.

There’s the how-to in WordPress Codex: http://codex.wordpress.org/Creating_Options_Pages, and there are also some plugins that do this, the problem is my “notes” are stored only for the site I’m currently on.

What is the way to build a plugin that would appear on all the subsites of multisite install, with the same content stored?

Thanks