README: mention scrypt_kdf()

Since the public library interface is scrypt_kdf(), we should include it
in the docs.

README.md

@@ -113,9 +113,11 @@ int crypto_scrypt(const uint8_t *, size_t, const uint8_t *, size_t, uint64_t,
     uint32_t, uint32_t, uint8_t *, size_t);
 ```
 
-The same function is provided in the optional `libscrypt-kdf` library; there
-is a sample of using it in `tests/libscrypt-kdf`.  If you installed the
-library, you can compile that file and run the binary:
+If you are using the optional `libscrypt-kdf` library, then please
+use the `scrypt_kdf()` function from `scrypt-kdf.h`, which has the
+same arguments as `crypto_scrypt()`.  There is a sample of using
+it in `tests/libscrypt-kdf`.  If you installed the library, you
+can compile that file and run the binary:
 
 ```
 $ cd tests/libscrypt-kdf/