Added URL to curve25519 paper.

misc/ecc-formulas.tex

@@ -231,8 +231,9 @@ Curve25519 is defined as the Montgomery curve
 \begin{equation*}
   y^2 = x^3 + b x^2 + x \pmod p
 \end{equation*}
-with $b = 486662$ and $p = 2^{255} -19$. It is equivalent to the
-Edwards curve
+with $b = 486662$ and $p = 2^{255} -19$. See
+\url{http://cr.yp.to/ecdh/curve25519-20060209.pdf}. It is equivalent
+to the Edwards curve
 \begin{equation*}
   u^2 + v^2 = 1 + d u^2 v^2 \pmod p
 \end{equation*}