Convert COPYING from rdoc to markdown.

This is part of an effort to make the documentation render correctly on
both Github and elsewhere.

Also use a standard markdown link to the Open Source Initiative MIT
license instead of an rdoc link to the local LICENSE file, because
the latter doesn't work on the Github rendering of the README.

COPYING.md

@@ -0,0 +1,3 @@
+Copyright Revieworld Ltd. 2006
+
+You may use, copy and redistribute this library under the same terms as [Ruby itself](http://www.ruby-lang.org/en/LICENSE.txt) or under the [MIT license](http://www.opensource.org/licenses/MIT).

README.md

@@ -114,4 +114,4 @@ Mocha was initially harvested from projects at [Reevoo](http://www.reevoo.com/).
 
 © Copyright Revieworld Ltd. 2006
 
-You may use, copy and redistribute this library under the same terms as [Ruby itself](http://www.ruby-lang.org/en/LICENSE.txt) or under the {file:MIT-LICENSE.rdoc MIT license}.
+You may use, copy and redistribute this library under the same terms as [Ruby itself](http://www.ruby-lang.org/en/LICENSE.txt) or under the [MIT license](http://www.opensource.org/licenses/MIT).

Rakefile

@@ -134,7 +134,7 @@ YARD::Rake::YardocTask.new('yardoc') do |task|
     'lib/mocha/integration/mini_test/adapter.rb',
     '-',
     'RELEASE.rdoc',
-    'COPYING.rdoc',
+    'COPYING.md',
     'MIT-LICENSE.rdoc',
     'examples/mocha.rb',
     'examples/stubba.rb',