Christopher Pitt posted a tutorial on creating composer plugins on the Sitepoint blog.

Composer is the sharpest tool in the toolbox of the modern PHP developer. The days of manual dependency management are in the distant past, and in their place we have wonderful things like Semver. ... Recent changes have made it much easier to develop Composer plugins.

http://www.sitepoint.com/drunk-with-the-power-of-composer-plugins/