Document hard challanges

[mitiko]

Challanges that were hard & require some visualization:

• 12 Byte-at-a-time ECB decryption (Simple)
• 13 ECB cut-and-paste
• 14 Byte-at-a-time ECB decryption (Harder)
• 16 CBC bitflipping attacks

Challanges that I think have cool rust code:

• 2 Fixed XOR
• 5 Implement repeating-key XOR
• 3 Single-byte XOR cipher
• 6 https://cryptopals.com/sets/1/challenges/6

Basically document all challanges but some can be short.
And clarify what's required.