Add documentation entries

doc/ci/variables/README.md

@@ -65,6 +65,8 @@ future GitLab releases.**
 | **CI_JOB_NAME**                 | 9.0    | 0.5    | The name of the job as defined in `.gitlab-ci.yml` |
 | **CI_JOB_STAGE**                | 9.0    | 0.5    | The name of the stage as defined in `.gitlab-ci.yml` |
 | **CI_JOB_TOKEN**                | 9.0    | 1.2    | Token used for authenticating with the [GitLab Container Registry][registry] and downloading [dependent repositories][dependent-repositories] |
+| **CI_NODE_INDEX**               | 11.5   | all    | The index of the job in the whole set. If the job is not paralellized, this is not set. |
+| **CI_NODE_TOTAL**               | 11.5   | all    | The total number of instances of this job running in parallel. If the job is not paralellized, this is set to 1. |
 | **CI_JOB_URL**                  | 11.1   | 0.5    | Job details URL |
 | **CI_REPOSITORY_URL**           | 9.0    | all    | The URL to clone the Git repository |
 | **CI_RUNNER_DESCRIPTION**       | 8.10   | 0.5    | The description of the runner as saved in GitLab |

doc/ci/yaml/README.md

@@ -75,6 +75,7 @@ A job is defined by a list of parameters that define the job behavior.
 | environment   | no       | Defines a name of environment to which deployment is done by this job |
 | coverage      | no       | Define code coverage settings for a given job |
 | retry         | no       | Define how many times a job can be auto-retried in case of a failure |
+| parallel      | no       | Define how many duplicates of a job should be run in parallel |
 
 ### `extends`
 
@@ -1451,6 +1452,28 @@ test:
   retry: 2
 ```
 
+## `parallel`
+
+> [Introduced][ce-22631] in GitLab 11.5.
+
+`parallel` allows you to configure how many duplicates of a job will be run in
+parallel. This value has to be greater or equal to two (2).
+
+This creates N duplicates of the same job that run in parallel. They're named
+sequentially from `job_name 1/N` to `job_name N/N`.
+
+For every job `CI_NODE_INDEX` and `CI_NODE_TOTAL` environment variables are set.
+`CI_NODE_TOTAL` represents the total number of jobs while `CI_NODE_INDEX` is the
+index of the job in the whole set.
+
+A simple example:
+
+```yaml
+test:
+  script: rspec
+  parallel: 5
+```
+
 ## `include`
 
 > Introduced in [GitLab Edition Premium][ee] 10.5.
@@ -2031,6 +2054,7 @@ CI with various languages.
 [ce-7983]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7983
 [ce-7447]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7447
 [ce-12909]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12909
+[ce-22631]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22631
 [schedules]: ../../user/project/pipelines/schedules.md
 [variables-expressions]: ../variables/README.md#variables-expressions
 [ee]: https://about.gitlab.com/gitlab-ee/