“Donate to this plugin” for WordPress.org Plugin Authors

It is read from the infos on your plugins readme.txt. Example from my Default Values for Attachments

=== Plugin Name ===

Contributors: moraleida.me

Donate link: http://moraleida.me/

Tags: attachments, default values, caption, title, description

Requires at least: 2.5 Tested up to: 3.3.2 Stable

tag: 0.1 License: GPLv2 or later License URI:

http://www.gnu.org/licenses/gpl-2.0.html