Merge pull request #9279 from ruby/add-colby

Added colby to maintainers

Author: hsbt

doc/MAINTAINERS.txt

@@ -1,3 +1,4 @@
 SHIBATA Hiroshi <hsbt@ruby-lang.org> (@hsbt)
 Aaron Patterson <tenderlove@ruby-lang.org> (@tenderlove)
 Sutou Kouhei <kou@clear-code.com> (@kou)
+Colby Swandale <colby@rubygems.org> (@colby-swandale)