Add example and update readme with redirect to PN532 library from seedstudio

https://github.com/Seeed-Studio/PN532 works very well with native SPI and it is also possible to combine the ETH shield with the PN532 shield by using two simple method to switch between the SPI configuration.
The Seeedstudio Library now also contains much more features. I think it would be best to inform visitors of this project that there is an alternativ library.

It is also not needed to bend any pins for the seedstudio NFC shield since there is a solder bridge which can be used to change SS to pin 9.