Publish deprecation notice

[kevinburkeotto]

Node.js has a timingSafeEqual function built in as of version 6.6.0: https://nodejs.org/api/crypto.html#crypto_crypto_timingsafeequal_a_b

It seems better to a) use that function if it's available and/or b) note in the README that people don't need to load a third party library to do that anymore.