add readme and move old one

README.md

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+## Description
+Fork of [ruby\_core\_source](https://github.com/mark-moseley/ruby_core_source)
+that uses included Ruby headers (\*.h and \*.inc) instead of downloading
+them. Used by [debugger](http://github.com/cldwalker/debugger).
+
+##Usage
+
+Example use in extconf.rb:
+
+```ruby
+require 'ruby_core_source'
+hdrs = proc { have_header("vm_core.h") and have_header("iseq.h") }
+dir_config("ruby") # allow user to pass in non-standard core include directory
+if !Ruby_core_source::create_makefile_with_core(hdrs, "foo")
+  # error
+  exit(1)
+end
+```
+
+To add another ruby version's source to this gem's directory:
+
+    $ rake add_source VERSION=1.9.3-p0
+
+## LICENSE
+Ruby library code is MIT license, see LICENSE.  Included ruby headers,
+lib/debugger/ruby\_core\_source/, are mostly Ruby license, see RUBY\_LICENSE. Some headers have
+their own licenses, see LEGAL.