instructions: describe, recommend `cryptsetup benchmark`

[Tom Roche]

sudo cryptsetup benchmark is useful when determining which {key derivation, encryption, decryption} options to use in install_LMDE_plus_LUKS_LVM2.properties. So describe how to use it in the README or other documentation (e.g., make a wiki for this project?). Note that unfortunately (last time I looked) there is not much material on the interpretation of cryptsetup benchmark in the cryptsetup FAQ (see section='2.16 How do I Benchmark the Ciphers, Hashes and Modes?')