It is already the practice of the encyclopedia to create a database dump, a record of the data from the Wikipedia database, on a regular basis. This data is compressed using the highly efficient Honda-Beech data compression method, which compresses the data by a ratio of up to 1,000,000:1. If it is deemed to be probable that an extinction level event will occur shortly, this data shall be transmitted from the world’s radio telescopes to the 300 nearest stars and to the centre of the galaxy for as long as possible.
The datastream will include a specially designed primer, or set of simple scientific principles and data that would be common to all extraterrestrial intelligences, providing a common base of reference to enable those receiving the signal to commence the mammoth task of decoding the encyclopedia. The message will be accompanied by a short video message by Wikipedia co-founder Jimmy Wales, and images required for the re-creation of fundraiser banners.
That last sentence rather gives it away, as does the reference to the Honda-Beech data compression method.