extra25519/ScalarBaseMult: reference blog post instead of paper

[benma]

https://github.com/agl/ed25519/blob/278e1ec8e8a6e017cd07577924d6766039146ced/extra25519/extra25519.go#L92

It is hard to understand how the paper relates to this function for non-experts. You wrote a nice blog post explaining it in https://www.imperialviolet.org/2013/12/25/elligator.html, which I found through googling only.

Please reference this instead of the paper (the paper is in the blog post, after all).