Port 'include' in CI yaml docs from EE

doc/ci/yaml/README.md

@@ -1354,6 +1354,187 @@ test:
   retry: 2
 ```
 
+## `include`
+
+> Introduced in [GitLab Edition Premium][ee] 10.5.
+> Available for Starter, Premium and Ultimate [versions][gitlab-versions] since 10.6.
+> Behaviour expanded in GitLab 10.8 to allow more flexible overriding.
+> Available for Libre since [11.4](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21603)
+
+Using the `include` keyword, you can allow the inclusion of external YAML files.
+
+In the following example, the content of `.before-script-template.yml` will be
+automatically fetched and evaluated along with the content of `.gitlab-ci.yml`:
+
+```yaml
+# Content of https://gitlab.com/awesome-project/raw/master/.before-script-template.yml
+
+before_script:
+  - apt-get update -qq && apt-get install -y -qq sqlite3 libsqlite3-dev nodejs
+  - gem install bundler --no-ri --no-rdoc
+  - bundle install --jobs $(nproc)  "${FLAGS[@]}"
+```
+
+```yaml
+# Content of .gitlab-ci.yml
+
+include: 'https://gitlab.com/awesome-project/raw/master/.before-script-template.yml'
+
+rspec:
+  script:
+    - bundle exec rspec
+```
+
+You can define it either as a single string, or, in case you want to include
+more than one files, an array of different values . The following examples
+are both valid cases:
+
+```yaml
+# Single string
+
+include: '/templates/.after-script-template.yml'
+```
+
+```yaml
+# Array
+
+include:
+  - 'https://gitlab.com/awesome-project/raw/master/.before-script-template.yml'
+  - '/templates/.after-script-template.yml'
+```
+
+---
+
+`include` supports two types of files:
+
+- **local** to the same repository, referenced by using full paths in the same
+  repository, with `/` being the root directory. For example:
+
+    ```yaml
+    # Within the repository
+    include: '/templates/.gitlab-ci-template.yml'
+    ```
+
+    NOTE: **Note:**
+    You can only use files that are currently tracked by Git on the same branch
+    your configuration file is. In other words, when using a **local file**, make
+    sure that both `.gitlab-ci.yml` and the local file are on the same branch.
+
+    NOTE: **Note:**
+    We don't support the inclusion of local files through Git submodules paths.
+
+- **remote** in a different location, accessed using HTTP/HTTPS, referenced
+  using the full URL. For example:
+
+    ```yaml
+    include: 'https://gitlab.com/awesome-project/raw/master/.gitlab-ci-template.yml'
+    ```
+
+    NOTE: **Note:**
+    The remote file must be publicly accessible through a simple GET request, as we don't support authentication schemas in the remote URL.
+
+---
+
+
+Since GitLab 10.8 we are now recursively merging the files defined in `include`
+with those in `.gitlab-ci.yml`. Files defined by `include` are always
+evaluated first and recursively merged with the content of `.gitlab-ci.yml`, no
+matter the position of the `include` keyword. You can take advantage of
+recursive merging to customize and override details in included CI
+configurations with local definitions.
+
+The following example shows specific YAML-defined variables and details of the
+`production` job from an include file being customized in `.gitlab-ci.yml`.
+
+```yaml
+# Content of https://company.com/autodevops-template.yml
+
+variables:
+  POSTGRES_USER: user
+  POSTGRES_PASSWORD: testing_password
+  POSTGRES_DB: $CI_ENVIRONMENT_SLUG
+
+production:
+  stage: production
+  script:
+    - install_dependencies
+    - deploy
+  environment:
+    name: production
+    url: https://$CI_PROJECT_PATH_SLUG.$AUTO_DEVOPS_DOMAIN
+  only:
+    - master
+```
+
+```yaml
+# Content of .gitlab-ci.yml
+
+include: 'https://company.com/autodevops-template.yml'
+
+image: alpine:latest
+
+variables:
+  POSTGRES_USER: root
+  POSTGRES_PASSWORD: secure_password
+
+stages:
+  - build
+  - test
+  - production
+
+production:
+  environment:
+    url: https://domain.com
+```
+
+In this case, the variables `POSTGRES_USER` and `POSTGRES_PASSWORD` along
+with the environment url of the `production` job defined in
+`autodevops-template.yml` have been overridden by new values defined in
+`.gitlab-ci.yml`.
+
+NOTE: **Note:**
+Recursive includes are not supported meaning your external files
+should not use the `include` keyword, as it will be ignored.
+
+Recursive merging lets you extend and override dictionary mappings, but
+you cannot add or modify items to an included array. For example, to add
+an additional item to the production job script, you must repeat the
+existing script items.
+
+```yaml
+# Content of https://company.com/autodevops-template.yml
+
+production:
+  stage: production
+  script:
+    - install_dependencies
+    - deploy
+```
+
+```yaml
+# Content of .gitlab-ci.yml
+
+include: 'https://company.com/autodevops-template.yml'
+
+stages:
+  - production
+
+production:
+  script:
+    - install_depedencies
+    - deploy
+    - notify_owner
+```
+
+In this case, if `install_dependencies` and `deploy` were not repeated in
+`.gitlab-ci.yml`, they would not be part of the script for the `production`
+job in the combined CI configuration.
+
+NOTE: **Note:**
+We currently do not support using YAML aliases across different YAML files
+sourced by `include`. You must only refer to aliases in the same file. Instead
+of using YAML anchors you can use [`extends` keyword](#extends).
+
 ## `variables`
 
 > Introduced in GitLab Runner v0.5.0.