Update preface.asciidoc with Luke Schoen as contributor

Propose to add Luke Schoen (ltfschoen) as contributor:
* 23 April 2018 - Offered addition of an Ethereum EVM Context diagram ethereumbook#496
 * 10 Mar - 14 Apr 2018 - See various Pull Requests that were offered https://github.com/ethereumbook/ethereumbook/commits?author=ltfschoen

Author: ltfschoen

preface.asciidoc

@@ -200,6 +200,7 @@ Following is an alphabetically sorted list of notable GitHub contributors, inclu
 * Jon Ramvi (ramvi)
 * Kevin Carter (kcar1)
 * Krzysztof Nowak (krzysztof)
+* Luke Schoen (ltfschoen)
 * Liang Ma (liangma)
 * Martin Berger (drmartinberger)
 * Matthew Sedaghatfar (sedaghatfar)