Add doc for 'label' and 'context' params in HKDF

[patrickfav]

These are according to the HKDF spec: https://tools.ietf.org/html/rfc5869

There is a extract and a expand phase. The salt (aka label) is used in
the extract phase:

HKDF-Extract(salt, IKM) -> PRK
salt     optional salt value (a non-secret random value);
         if not provided, it is set to a string of HashLen zeros.

and the info (aka context) is used in the expand phase:

HKDF-Expand(PRK, info, L) -> OKM
info     optional context and application specific information
         (can be a zero-length string)

[patrickfav]

Any chance this will be merged?

[LiraNuna]

Please note that salt and info are required as per webcrypto spec: https://www.w3.org/TR/WebCryptoAPI/#hkdf-params

In addition, HKDF-CRT was replaced with HKDF, and I already addressed it with #54

[patrickfav]

Thanks for the heads up. Thats interesting as the the RFC defines it as optional. Your PR is probably better then. Unfortunately it seems the maintainer has no interest in merging these PRs anytime soon.