README.EXT: preliminary documentation for RB_GC_GUARD

[Bug #10100] [ruby-core:60741]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47180 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-08-15

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47181 b2dd03c8-39d4-4d8f-98ff-823fe69b080e