add description for what how the vanishing polynomial is calculated as X^N - 1

doc/vanishing-poly.md

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+We have a vanishing polynomial $Z(X) = X^N - 1$. Implicitly we are proving that
+
+$$X^N = 1$$
+
+What are the solutions to this polynomial? Well the answer is $\omega$ which is any root of $1$.
+
+Therefore the solution to the formula $X^N - 1$ will be all the values of $X^N = 1$ or
+
+$$X^N - 1 = (\omega - 1)(\omega^2 - 1)\cdots(\omega^{N - 1} - 1)$$
+