Update note about tested Ruby versions [skip ci]

Author: jgarber623

CONTRIBUTING.md

@@ -8,7 +8,7 @@ There are a couple ways you can help improve indieweb-endpoints-ruby:
 
 ## Getting Started
 
-indieweb-endpoints-ruby is developed using Ruby 2.5.9 and is additionally tested against Ruby 2.6 and 2.7 using [Travis CI](https://travis-ci.com/indieweb/indieweb-endpoints-ruby).
+indieweb-endpoints-ruby is developed using Ruby 2.5.9 and is additionally tested against Ruby 2.6, 2.7, and 3.0 using [Travis CI](https://travis-ci.com/indieweb/indieweb-endpoints-ruby).
 
 Before making changes to indieweb-endpoints-ruby, you'll want to install Ruby 2.5.9. It's recommended that you use a Ruby version managment tool like [rbenv](https://github.com/rbenv/rbenv), [chruby](https://github.com/postmodern/chruby), or [rvm](https://github.com/rvm/rvm). Once you've installed Ruby 2.5.9 using your method of choice, install the project's gems by running: