README.EXT: add redirect [ruby-core:68631]

There are too many 3rd-party documentation sources which refer
to README.EXT, so we should ensure readers can find the file at
the new location.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50090 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,7 @@
+Thu Mar 26 06:35:10 2015  Eric Wong  <e@80x24.org>
+
+	* README.EXT: add redirect [ruby-core:68631]
+
 Wed Mar 25 16:46:49 2015  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* ext/socket/extconf.rb: Solaris 11 has struct tcp_info.tcpi_ca_state,

README.EXT

@@ -0,0 +1 @@
+Moved to doc/extension.rdoc