add 'Cryptohack'

- signed-off-by: trimstray <trimstray@gmail.com>

README.md

@@ -1416,6 +1416,7 @@ AWS deployment tool.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="https://capturetheflag.withgoogle.com"><b>gCTF</b></a> - most of the challenges used in the Google CTF 2017.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="https://www.hackthissite.org/pages/index/index.php"><b>Hack This Site</b></a> - is a free, safe and legal training ground for hackers.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="https://attackdefense.com"><b>Attack & Defense</b></a> - is a browser-based cloud labs.<br>
+&nbsp;&nbsp;:small_orange_diamond: <a href="https://cryptohack.org/"><b>Cryptohack</b></a> - a fun platform for learning modern cryptography.<br>
 </p>
 
 ##### :black_small_square: CTF platforms