Document custom build directories

Update all references to version to include `v` for consistency.

Closes gitlab-org/gitlab-runner#2211

doc/ci/yaml/README.md

@@ -1406,6 +1406,43 @@ variables:
 You can set it globally or per-job in the [`variables`](#variables) section.
+### Custom build directories
+
+> [Introduced][gitlab-runner-876] in Gitlab Runner 11.1
+
+NOTE: **Note:**
+This can only be used when `custom_build_dir` is set to true in the [Runner's
+configuration](https://docs.gitlab.com/runner/configuration/advanced-configuration.html).
+
+By default, GitLab Runner clones the repository in the `/builds` directory,
+but sometimes your project might require to have the code in a specific
+directory, like the GO projects for example. In that case, you can specify
+the `CI_PROJECT_DIR` variable to tell the Runner in which directory to clone
+the repository:
+
+```yml
+image: golang:1.10-alpine3.7
+
+variables:
+  CI_PROJECT_DIR: /go/src/gitlab.com/namespace/project-name
+
+stages:
+    - test
+
+dir:
+    stage: test
+    script:
+      - pwd # /go/src/gitlab.com/namespace/project-name
+```
+
+The following executors may use this feature only when
+[concurrent](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-global-section)
+is set to `1`:
+
+- `shell`
+- `ssh`
+- `docker`, `docker+machine` when the job's working directory is mounted as a host volume.
+
 ## Special YAML features
 It's possible to use special YAML features like anchors (`&`), aliases (`*`)
@@ -1599,5 +1636,6 @@ 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
+[gitlab-runner-876]: https://gitlab.com/gitlab-org/gitlab-runner/merge_requests/876
 [schedules]: ../../user/project/pipelines/schedules.md
 [variables-expressions]: ../variables/README.md#variables-expressions