Update README section on writing tests

* Add new section on how to add new dependency group

docs/DevelopmentGuide.md

@@ -44,34 +44,6 @@ New tests should be written as RSpec tests in the `spec/datadog` folder. Test fi
 
 All changes should be covered by a corresponding RSpec tests. Unit tests are preferred, and integration tests are accepted where appropriate (e.g. acceptance tests, verifying compatibility with datastores, etc) but should be kept to a minimum.
 
-**Considerations for CI**
-
-All tests should run in CI. When adding new `_spec.rb` files, you may need to add rake task to ensure your test file is run in CI.
-
- - Ensure that there is a corresponding Rake task defined in `Rakefile` under the `spec` namespace, whose pattern matches your test file. For example
-
- ```ruby
-   namespace :spec do
-     RSpec::Core::RakeTask.new(:foo) do |t, args|
-       t.pattern = "spec/datadog/tracing/contrib/foo/**/*_spec.rb"
-       t.rspec_opts = args.to_a.join(' ')
-     end
-   end
- ```
-
- - Ensure the Rake task is configured to run for the appropriate Ruby runtimes, by adding it to our `Matrixfile`. You should find the task with `bundle exec rake -T test:foo` after adding it.
-
-```ruby
-  {
-    'foo' => {
-      # With default dependencies for each Ruby runtime
-      ''    => '✅ 2.1 / ✅ 2.2 / ✅ 2.3 / ✅ 2.4 / ✅ 2.5 / ✅ 2.6 / ✅ 2.7 / ✅ 3.0 / ✅ 3.1 / ✅ 3.2 / ✅ 3.3 / ✅ 3.4 / ✅ jruby',
-      # or with dependency group definition `foo-on-rails`, that includes additional gems
-      'foo-on-rails' => '✅ 2.1 / ✅ 2.2 / ✅ 2.3 / ✅ 2.4 / ✅ 2.5 / ✅ 2.6 / ✅ 2.7 / ✅ 3.0 / ✅ 3.1 / ✅ 3.2 / ✅ 3.3 / ✅ 3.4 / ✅ jruby'
-    },
-  }
-```
-
 ### Running tests
 
 `bundle exec rake ci` will run the entire test suite with any given Ruby runtime, just as CI does. However, this is not recommended because it is going take a long time.
@@ -149,6 +121,83 @@ bundle exec rake dependency:lock['/app/gemfiles/ruby_3.3_stripe_*.gemfile']
 bundle exec rake dependency:lock['/app/gemfiles/ruby_3.3_stripe_latest.gemfile']
 ```
 
+**How to add new dependency group**
+
+1. Pick the Ruby runtime and group name you want your tests to run. When defining a new group, try to match the format `scope:group`.
+Let's say you want it to run only on Ruby 3.3 for profiling, than you can define it in the [`Matfixfile`](../Matrixfile).
+
+```ruby
+{
+  'tracing:ruby-on-rails' => {
+    # With default dependencies for each Ruby runtime
+    '' => '❌ 2.5 / ❌ 2.6 / ❌ 2.7 / ❌ 3.0 / ❌ 3.1 / ❌ 3.2 / ✅ 3.3 / ❌ 3.4 / ❌ jruby'
+    # or with dependency group definition `ruby-on-rails`, that includes additional gems or specific versions
+    'rails-1' => '❌ 2.5 / ❌ 2.6 / ❌ 2.7 / ❌ 3.0 / ❌ 3.1 / ❌ 3.2 / ✅ 3.3 / ❌ 3.4 / ❌ jruby'
+    'rails-edge' => '❌ 2.5 / ❌ 2.6 / ❌ 2.7 / ❌ 3.0 / ❌ 3.1 / ❌ 3.2 / ✅ 3.3 / ❌ 3.4 / ❌ jruby'
+    # ...
+  }
+}
+```
+
+2. Define the required gems in corresponding Appraisal file, in our case it is [`Appraisal/ruby-3.3.rb`](../Appraisal/ruby-3.3.rb). Let's define what is `rails-edge` group means.
+
+```ruby
+appraise 'rails-42' do
+  gem 'rails', '>= 8'
+end
+```
+
+3. Now let's generate that dependency Gemfile with `rake`, simply run
+
+> [!IMPORTANT] Make sure, that you are running all commands with Ruby 3.3 as current ruby version `ruby -v` or in Docker container
+
+```console
+$ bundle exec rake dependency:generate
+...
+ruby-3.3_rails-edge
+```
+
+and you can verify that this dependency now is in the list
+
+```console
+$ bundle exec rake dependency:list
+Ahoy! Here is a list of gemfiles you are looking for:
+
+========================================
+...
+/Users/DataDog/dd-trace-rb/gemfiles/ruby_3.3_rails_edge.gemfile
+```
+
+4. Lock the gems versions with
+
+```console
+$ bundle exec rake dependency:lock[]
+BUNDLE_GEMFILE=/Users/DataDog/dd-trace-rb/gemfiles/ruby_3.3_rails_edge.gemfile bundle lock --add-platform x86_64-linux aarch64-linux
+Fetching gem metadata from https://rubygems.org/...........
+Resolving dependencies...
+Writing lockfile to /Users/DataDog/dd-trace-rb/gemfiles/ruby_3.3_rails_edge.gemfile.lock
+```
+
+5. The last step is to associate the newly generated group with some tests. It can be done in the [`Rakefile`](../Rakefile). It is important that the `scope:group` format of the new group match the rake task name. In our case, we should define it like this
+
+```ruby
+namespace :spec do
+  namespace :tracing do
+    RSpec::Core::RakeTask.new(:foo) do |t, args|
+      t.pattern = "spec/datadog/tracing/contrib/ruby-on-rails/**/*_spec.rb"
+      t.rspec_opts = args.to_a.join(' ')
+    end
+  end
+end
+```
+
+and now you should be able to find it by running
+
+```console
+$ bundle exec rake -T test:tracing
+rake test:tracing:ruby-on-rails[task_args]  # Run spec:tracing:ruby-on-rails tests
+```
+
 **Passing arguments to tests**
 
 When running tests, you may pass additional args as parameters to the Rake task. For example: