Add CONTRIBUTING.md with basic setup

Author: trevorrjohn

CONTRIBUTING.md

@@ -0,0 +1,31 @@
+# Contributing
+
+## How to contribute
+
+This gem uses [Appraisal](https:..//github.com/thoughtbot/appraisal) to test against multiple versions of Rails.
+
+Some versions of Ruby are not supported using different versions of Rails. Checkout
+the .github/workflows/ci.yml and Appraisals file to see which versions of Ruby are tested.
+
+## Running tests
+
+Install dependencies:
+
+```bash
+bundle install
+bundle exec appraisal bundle install
+```
+
+Both Appraisals and ci.yml define what versions of Ruby and Rails are supported.
+
+Run tests for all Rails versions supported for your Ruby version:
+
+```bash
+bundle exec appraisal bundle exec rake
+```
+
+Run tests against a specific version of Rails:
+
+```bash
+bundle exec appraisal rails_main bundle exec rake
+```