Adding Library to my sketchbook
Download https://github.com/BoolLi/RC522_RFID_NFC_Utilities/archive/master.zip
Copy the folder RC522_RFID_Utilities from the downloaded file to {sketchbook folder}/libraries. You can find the location of your sketchbook folder in the Arduino IDE at File > Preferences > Sketchbook location.
If you're using Arduino IDE 1.6.5-r5 or older restart the Arduino IDE. If using a newer version no restart is necessary.


The other three folders in the downloaded file are example sketches, not libraries. After installing the RC522_RFID_NFC_Utilities library following the instructions above you will be able to use them. You can store those folders anywhere you like but you should not put them directly under the libraries folder as this will cause warning messages about invalid libraries on every compilation. If you do want to bundle them with the RC522_RFID_NFC_Utilities library, as is customary, you can do this:


Create a folder named examples in {sketchbook folder}/libraries/RC522_RFID_NFC_Utilities
Copy the example sketch folders (NDEFFormattingExample, etc) from the downloaded file to the examples folder you created.
Restart the Arduino IDE if it's running.


The example sketches will now be accessible via File > Examples > RC522_RFID_NFC_Utilities.