Today I am writing the equivalent of this article for add-ins that I wrote originally almost 6 years ago:

HOWTO: Get an OutputWindowPane to output some string from a Visual Studio add-in or macro
http://www.mztools.com/articles/2008/MZ2008023.aspx

but in this case natively (using services) for Visual Studio packages:

HOWTO: Write to the output window panes from a Visual Studio package.
http://www.mztools.com/articles/2014/MZ2014017.aspx