Updated the ReadAndWrite example

Updated the `ReadAndWrite.ino` example sketch/program:

* Changed the description, to show that it is an example for the MFRC522 library
* Added link to the GitHub based project for the MFRC522 library
* Expanded the description of the example a little; including warning that data will be written
* Updated the pin layout based on #48 updated README
* Changed order (read, write, read/check) and added more comments to explain the process a little better
* Removed some stuff that had to do with value blocks, where this is just data in a block

Using block 4 in this example, in sector 1 (for this `ReadAndWrite.ino` example), which is compatible with the `MifareClassicValueBlock.ino` example (using block 5 and 6 of sector 1). Both examples use trailer block 7, though this `ReadAndWrite.ino` example is not trying to modify it.