Added maintainers of embedded core classes

Author: hsbt

doc/maintainers.md

@@ -50,7 +50,7 @@ consensus on ruby-core/ruby-dev.
 
 * *No maintainer*
 
-#### lib/pathname.rb
+#### pathname_builtin.rb, lib/pathname.rb
 
 * Tanaka Akira ([akr])
 
@@ -65,6 +65,10 @@ consensus on ruby-core/ruby-dev.
 
 ### Standard Library(Extensions) Maintainers
 
+#### set.c
+
+* Akinori MUSHA ([knu])
+
 #### ext/continuation
 
 * Koichi Sasada ([ko1])
@@ -85,6 +89,10 @@ consensus on ruby-core/ruby-dev.
 
 * *No maintainer*
 
+#### ext/pathname
+
+* Tanaka Akira ([akr])
+
 #### ext/pty
 
 * *No maintainer*