README: mention scrypt_kdf()

[gperciva]

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[gperciva]

Partially addresses #277 (if this is accepted, I'll update the website accordingly).

Original discussion about the public header for libscrypt-kdf is here:
#168 (comment)

My first instinct was to remove all mentions of crypto_scrypt() from that section, but historically people have used scrypt by copying files into their own project and calling that function. I don't know if we should try to discourage that now, so I opted for a "minimal" change of mentioning scrypt_kdf() in the context of the optional libscrypt-kdf.

[gperciva]

Updated the PR to mention scrypt_kdf() first, and significantly downplay the "copy source files into your project" option. See it in action:
https://github.com/Tarsnap/scrypt/tree/discuss-scrypt-kdf#using-scrypt-as-a-kdf