Adds Contributing.md (#211)

* don't run appraisal's for unsupported ruby versions

* Add CONTRIBUTING.md with basic setup

Author: trevorrjohn

Appraisals

@@ -1,12 +1,20 @@
-appraise "rails_6_1" do
-  gem "rails", "~> 6.1.0"
-  gem 'concurrent-ruby', '<= 1.3.4'
+RUBY_MAJOR_VERSION, RUBY_MINOR_VERSION, _ = RUBY_VERSION.split(".").map(&:to_i)
+
+# run on ruby <= 3.2
+if RUBY_MAJOR_VERSION == 3 && RUBY_MINOR_VERSION <= 2
+  appraise "rails_6_1" do
+    gem "rails", "~> 6.1.0"
+    gem 'concurrent-ruby', '<= 1.3.4'
+  end
 end
 
-appraise "rails_7_0" do
-  gem "rails", "~> 7.0.0"
-  gem 'concurrent-ruby', '<= 1.3.4'
-  gem "propshaft"
+# run on ruby <= 3.3
+if RUBY_MAJOR_VERSION == 3 && RUBY_MINOR_VERSION <= 3
+  appraise "rails_7_0" do
+    gem "rails", "~> 7.0.0"
+    gem 'concurrent-ruby', '<= 1.3.4'
+    gem "propshaft"
+  end
 end
 
 appraise "rails_7_1" do
@@ -19,12 +27,15 @@ appraise "rails_7_2" do
   gem "propshaft"
 end
 
-appraise "rails_8_0" do
-  gem "rails", "~> 8.0.0"
-  gem "propshaft"
-end
+# run on ruby >= 3.1
+if RUBY_MAJOR_VERSION >= 3 && RUBY_MINOR_VERSION > 1
+  appraise "rails_8_0" do
+    gem "rails", "~> 8.0.0"
+    gem "propshaft"
+  end
 
-appraise "rails_main" do
-  gem "rails", github: "rails/rails", branch: "main"
-  gem "propshaft"
+  appraise "rails_main" do
+    gem "rails", github: "rails/rails", branch: "main"
+    gem "propshaft"
+  end
 end

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
+```