Added reference to platform specific cryptographic acceleration such as AES-NI

Where possible the B<EVP> interface to symmetric ciphers should be used in

preference to the low level interfaces. This is because the code then becomes

transparent to the cipher used and much more flexible.

transparent to the cipher used and much more flexible. Additionally, the

B<EVP> interface will ensure the use of platform specific cryptographic

acceleration such as AES-NI (the low level interfaces do not provide the

guarantee).

PKCS padding works by adding B<n> padding bytes of value B<n> to make the total 

length of the encrypted data a multiple of the block size. Padding is always