Updated the Mifare Classic Value Block example

Updated the `MifareClassicValueBlock.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
* Added more comments to explain the process a little better (especially Access Bits stuff)
* Added a `mifare_GetValue` helper routine to read the current value from a Value Block
* Added a `mifare_SetValue` helper routine to write a specific value into a Value Block
* Added decrement example using `valueBlockB` (= block #6 within sector #1 in this sample)
* Added check for Value Block dropping below a value, and (just for fun) resetting it to a specific value; see the `valueBlockB` part: decrementing by 10 each time PICC is presented, until it drops below -100; at which time it is set to 0xFF = 255 (just an arbitrary value which is easy to check in Serial dump)