Docs: Added fallback_cache_key

Related to: https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/1534

doc/ci/yaml/README.md

@@ -2819,6 +2819,8 @@ cache:
     - binaries/
 ```
 
+You can specify a [fallback cache key](#fallback-cache-key) to use if the specified `cache:key` is not found.
+
 ##### `cache:key:files`
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/18986) in GitLab v12.5.
@@ -4404,6 +4406,32 @@ variables:
 
 You can set them globally or per-job in the [`variables`](#variables) section.
 
+### Fallback cache key
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/1534) in GitLab Runner 13.4.
+
+You can use the `$CI_COMMIT_REF_SLUG` variable to specify your [`cache:key`](#cachekey).
+For example, if your `$CI_COMMIT_REF_SLUG` is `test` you can set a job
+to download cache that's tagged with `test`.
+
+If a cache with this tag is not found, you can use `CACHE_FALLBACK_KEY` to 
+specify a cache to use when none exists.
+
+For example:
+
+```yaml
+variables:
+  CACHE_FALLBACK_KEY: fallback-key
+
+cache:
+  key: "$CI_COMMIT_REF_SLUG"
+  paths:
+    - binaries/
+```
+
+In this example, if the `$CI_COMMIT_REF_SLUG` is not found, the job uses the key defined
+by the `CACHE_FALLBACK_KEY` variable.
+
 ### Shallow cloning
 
 > Introduced in GitLab 8.9 as an experimental feature.