add Book of Proof by Richard Hammack (#2025)

free-programming-books.md

@@ -395,6 +395,7 @@
 * [Advanced Algebra](http://www.math.stonybrook.edu/~aknapp/download/a2-alg-inside.pdf) - Anthony W. Knapp (PDF)
 * [Basic Algebra](http://www.math.stonybrook.edu/~aknapp/download/b2-alg-inside.pdf) - Anthony W. Knapp (PDF)
 * [Bayesian Methods for Hackers](https://github.com/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers) - Cameron Davidson-Pilon
+* [Book of Proof](http://www.people.vcu.edu/~rhammack/BookOfProof/) - Richard Hammack (PDF)
 * [Calculus Made Easy](http://www.gutenberg.org/ebooks/33283) - Silvanus P. Thompson (PDF)
 * [Category Theory for the Sciences](http://category-theory.mitpress.mit.edu)
 * [CK-12 Probability and Statistics - Advanced](http://www.ck12.org/book/Probability-and-Statistics---Advanced-%2528Second-Edition%2529/)