merge revision(s) 44374,44375,44376: [Backport #9290]

	* README.EXT: add a document about RGenGC.
	  Reviewed by havenwood.
	  [misc #8962]

	* README.EXT.ja: ditto.

	* README.EXT: add a refer to URL.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@44397 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-12-25

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@44398 b2dd03c8-39d4-4d8f-98ff-823fe69b080e