Adding RFID-RC522 Pin Layout image

Adding a RFID-RC522 Pin Layout image to the `doc` folder, based on the RFID-RC522-v2 module added in #54 (and #50). It is inspired by https://github.com/r00tGER/RFID-RC522, but this is my own work: the Fritzing part was derived from Peremptor's work as described in #50; after which I made a PNG export (see #54), modified the image a bit while added text labels to components and pins (using Paint.NET and looking at my own module) and then added the pin labels (in a Google Docs presentation) based on @r00tGER 's work and the Pin Layout section in the MFRC522 README file.