Update credits

Add all authors since the last update.
From the git history:
git log --format="%aN" fbc48e4f73d17f3014d9865836d21d6bdcfe9ebd..HEAD | sort -u

CREDITS.md

@@ -62,6 +62,7 @@
 * No Face Press
 * Paul Sokolovsky
 * Perttu Ahola
+* Peter Foerster
 * Philipp Friedenberger
 * Philipp Hasper
 * Red54
@@ -79,6 +80,7 @@
 * Ulrich Hertlein
 * Walt Steverson
 * William Greathouse
+* xtne6f
 * Yehuda Sadeh
 
 # Mongoose Contributors