There are actually two fonctionnal ways to connect a Raspberry PI to your Amstrad CPC :

• CPC-Cplink by Revaldinho, see the corresponding wiki and the github page. The card provides an easy to use parallel interface between the processors using First-In First-Out (FIFO) hardware and a simple message passing software protocol
• the all new Soyuz by Abalore who is very active at the moment (github of Soyuz)

There is also another WIP project by Zhulien : the CPC / Raspberry Pi Bridge Card, but for more recent informations you must read the thread Raspberry PI on CPC.