A Raspberry Pi Zero Shield that will connect a Zero or Zero W module to a 3.3V or 5V compatible Arduino board, is available from US-based Burgessworld Custom Electronics.
The two boards are connected via UART, SPI, I2C or logic-level pin signalling.
And there are current limiting resistors to protect the Raspberry Pi’s inputs.
The shield can be powered using the onboard 2.1mm barrel jack or micro USB port and offers support for up to 2.5A of current.
As described on the Hackster Profar blog, as features go, the Shield sports a reset switch, power LED, Arduino pin 13 LED, Pi GPIO pin 13 LED, headers for full Arduino pin access, and ICSP header for AVR microcontrollers.