CONTRIBUTING.md in rmagick-2.16.0 vs CONTRIBUTING.md in rmagick-3.0.0

@@ -20,10 +20,11 @@
 Priorities
 ----------
 
 1. Green build of the gem on all operating systems. You can see the current build state on [the project page at Travis CI](https://travis-ci.org/rmagick/rmagick). You are welcome to improve it.
 2. [Open issues](https://github.com/rmagick/rmagick/issues). You are welcome to reproduce them, report current state, suggest solutions, open pull requests with fixes. If you don't know where to start, sort issues by least recently updated. You can also [subscribe to receive random issues by email](http://www.codetriage.com/rmagick/rmagick).
+3. [CodeClimate Issues](https://codeclimate.com/github/rmagick/rmagick/issues). You can install [CodeClimate Browser Extension](https://docs.codeclimate.com/docs/browser-extension) to see them right on GitHub.
 
 
 Testing
 -------
 
@@ -31,11 +32,21 @@
 
 If you write new tests, please do it in RSpec.
 
 You are also welcome to convert existing Test/Unit tests to RSpec.
 
+Run all tests:
 
+    bundle exec rake
+
+It will run RSpec and Minitest tests.
+
+Useful information about RSpec:
+* [Better Specs](http://www.betterspecs.org/) — how to write RSpec tests better
+* [RSpec matchers cheat sheet](http://cheatrags.com/rspec-matchers) - how to use RSpec matchers to test different things
+
+
 Committing
 ----------
 
 All work for a next release is done in the `develop` branch. Please create your branch off of it.
 
@@ -46,5 +57,28 @@
 -------------
 
 Please choose the `rmagick/rmagick` repo and the `develop` branch as the destination for your pull request.
 
 NOTE: GitHub suggests `rmagick-temp/rmagick` repo by default. **This is incorrect.** Please switch to `rmagick/rmagick`. It should be the next repo in the drop-down list.
+
+Every pull request is tested:
+
+1. On [Travis CI](https://travis-ci.org/rmagick/rmagick).
+   It runs all Minitest and RSpec tests on several Ruby and ImageMagick versions.
+   See [.travis.yml](.travis.yml) for details.
+2. On [Semaphore CI](https://semaphoreci.com/rmagick/rmagick).
+   It runs [RuboCop](https://rubocop.readthedocs.io).
+   RuboCop is run on a different CI server and appears as a different check on pull request pages because this way it is easier to spot what kind of tests are failed.
+
+An ideal pull request does not fail these tests.
+If your pull request fails these tests, please improve it and push the improvements until it doesn't.
+You can ignore failures that are not caused by your changes.
+
+A quick way to fix formatting errors is to run `bundle exec rubocop --autocorrect path/to/file.rb`.
+
+If you add new classes or methods, please add tests for them as well.
+RSpec is preferred (see above).
+Tests should test all possible branches of code execution (all conditions in `if`/`case` statements, etc.) to avoid errors later.
+
+
+Thanks
+------