Add credit for maintenance in Rake 12/13

Shows current maintainers to wider community contributors.

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Author: tnir

README.rdoc

@@ -116,7 +116,7 @@ other projects with similar (and not so similar) goals.
 
 [<b>Eric Hodel</b>] For aid in maintaining rake.
 
-[<b>Hiroshi SHIBATA</b>] Maintainer of Rake 10.X and Rake 11.X
+[<b>Hiroshi SHIBATA</b>] Maintainer of Rake 10 and later
 
 == License