Documentation: Added Semaphore Test Boosters example to CI YAML `parallel` docs

doc/ci/yaml/README.md

@@ -1771,18 +1771,41 @@ sequentially from `job_name 1/N` to `job_name N/N`.
 
 For every job, `CI_NODE_INDEX` and `CI_NODE_TOTAL` [environment variables](../variables/README.md#predefined-environment-variables) are set.
 
-A simple example:
+Marking a job to be run in parallel requires only a simple addition to your configuration file:
 
-```yaml
-test:
-  script: rspec
-  parallel: 5
+```diff
+ test:
+   script: rspec
++  parallel: 5
 ```
-
 TIP: **Tip:**
 Parallelize tests suites across parallel jobs.
 Different languages have different tools to facilitate this.
 
+A simple example using [Sempahore Test Boosters](https://github.com/renderedtext/test-boosters) and RSpec to run some Ruby tests:
+
+```ruby
+# Gemfile
+source 'https://rubygems.org'
+
+gem 'rspec'
+gem 'semaphore_test_boosters'
+```
+
+```yaml
+test:
+  parallel: 3
+  script:
+    - bundle
+    - bundle exec rspec_booster --job $CI_NODE_INDEX/$CI_NODE_TOTAL
+```
+
+CAUTION: **Caution:**
+Please be aware that semaphore_test_boosters reports usages statistics to the author.
+
+You can then navigate to the **Jobs** tab of a new pipeline build and see your RSpec
+job split into three separate jobs.
+
 ### `trigger` **(PREMIUM)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/8997) in [GitLab Premium](https://about.gitlab.com/pricing/) 11.8.