Merge pull request ondryaso#47 from stevenroh/master

Update datasheet pdf link

Author: ondryaso

README.md

@@ -16,7 +16,7 @@ python setup.py install
 ```
 You'll also need to install the [**spidev**](https://pypi.python.org/pypi/spidev) and [**RPi.GPIO**](https://pypi.python.org/pypi/RPi.GPIO) libraries on Raspberry PI, and [**Adafruit_BBIO**](https://github.com/adafruit/adafruit-beaglebone-io-python) on Beaglebone Black (which should be installed by default).
 
-[MIFARE datasheet](http://www.nxp.com/documents/data_sheet/MF1S503x.pdf) can be useful.
+[MIFARE datasheet](https://www.nxp.com/docs/en/data-sheet/MF1S50YYX_V1.pdf) can be useful.
 
 ## Sectors? Blocks?
 Classic 1K MIFARE tag has **16 sectors**, each contains **4 blocks**. Each block has 16 bytes. All this stuff is indexed - you must count from zero. The library uses "**block addresses**", which are positions of blocks - so block address 5 is second block of second sector, thus it's block 1 of sector 1 (indexes). Block addresses 0, 1, 2, 3 are from the first sector - sector 0. Block addresses 4, 5, 6, 7 are from the second sector - sector 1, and so on. You should **not write** to first block - S0B0, because it contains manufacturer data. Each sector has it's **sector trailer**, which is located at it's last block - block 3. This block contains keys and access bits for corresponding sector. For more info, look at page 10 of the datasheet. You can use [this](http://www.proxmark.org/forum/viewtopic.php?id=1408) useful utility to calculate access bits.