Merge branch 'document-allow_failure-keyword' into 'master'

Add allow_failure CI documentation

## What does this MR do?

Adds documentation for the `allow_failure` keyword for the CI.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/17669

See merge request !5398

doc/ci/yaml/README.md

@@ -27,12 +27,14 @@ If you want a quick introduction to GitLab CI, follow our
     - [only and except](#only-and-except)
     - [job variables](#job-variables)
     - [tags](#tags)
+    - [allow_failure](#allow_failure)
     - [when](#when)
+        - [Manual actions](#manual-actions)
     - [environment](#environment)
     - [artifacts](#artifacts)
-    - [artifacts:name](#artifactsname)
-    - [artifacts:when](#artifactswhen)
-    - [artifacts:expire_in](#artifactsexpire_in)
+        - [artifacts:name](#artifacts-name)
+        - [artifacts:when](#artifacts-when)
+        - [artifacts:expire_in](#artifacts-expire_in)
     - [dependencies](#dependencies)
     - [before_script and after_script](#before_script-and-after_script)
 - [Git Strategy](#git-strategy)
@@ -40,7 +42,7 @@ If you want a quick introduction to GitLab CI, follow our
 - [Hidden jobs](#hidden-jobs)
 - [Special YAML features](#special-yaml-features)
     - [Anchors](#anchors)
-- [Validate the .gitlab-ci.yml](#validate-the-gitlab-ciyml)
+- [Validate the .gitlab-ci.yml](#validate-the-gitlab-ci-yml)
 - [Skipping builds](#skipping-builds)
 - [Examples](#examples)
 
@@ -473,6 +475,39 @@ job:
 The specification above, will make sure that `job` is built by a Runner that
 has both `ruby` AND `postgres` tags defined.
 
+### allow_failure
+
+`allow_failure` is used when you want to allow a build to fail without impacting
+the rest of the CI suite. Failed builds don't contribute to the commit status.
+
+When enabled and the build fails, the pipeline will be successful/green for all
+intents and purposes, but a "CI build passed with warnings" message  will be
+displayed on the merge request or commit or build page. This is to be used by
+builds that are allowed to fail, but where failure indicates some other (manual)
+steps should be taken elsewhere.
+
+In the example below, `job1` and `job2` will run in parallel, but if `job1`
+fails, it will not stop the next stage from running, since it's marked with
+`allow_failure: true`:
+
+```yaml
+job1:
+  stage: test
+  script:
+  - execute_script_that_will_fail
+  allow_failure: true
+
+job2:
+  stage: test
+  script:
+  - execute_script_that_will_succeed
+
+job3:
+  stage: deploy
+  script:
+  - deploy_to_staging
+```
+
 ### when
 
 `when` is used to implement jobs that are run in case of failure or despite the