Update README references in gemspec and CONTRIBUTING.md

Author: filabreu

CONTRIBUTING.md

@@ -49,7 +49,7 @@ Make sure that `bundle exec rake test` completes without errors.
 
 #### Write Documentation
 
-Document any external behavior in the [README](README.rdoc).
+Document any external behavior in the [README](README.md).
 
 #### Commit Changes
 

activeresource.gemspec

@@ -15,12 +15,9 @@ Gem::Specification.new do |s|
   s.email       = "[email protected]"
   s.homepage    = "http://www.rubyonrails.org"
 
-  s.files = Dir["MIT-LICENSE", "README.rdoc", "lib/**/*"]
+  s.files = Dir["MIT-LICENSE", "README.md", "lib/**/*"]
   s.require_path = "lib"
 
-  s.extra_rdoc_files = %w( README.rdoc )
-  s.rdoc_options.concat ["--main",  "README.rdoc"]
-
   s.required_ruby_version = ">= 2.2.2"
 
   s.add_dependency("activesupport", ">= 5.0", "< 7")