Improve release-cli documentation

doc/ci/yaml/README.md

@@ -4101,6 +4101,7 @@ finishes.
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/merge_requests/19298) in GitLab 13.2.
 
 Use `release` to create a [release](../../user/project/releases/index.md).
+Requires the `release-cli` to be available in your GitLab Runner Docker or shell executor.
 
 These keywords are supported:
 
@@ -4122,6 +4123,69 @@ You must specify the Docker image to use for the `release-cli`:
 image: registry.gitlab.com/gitlab-org/release-cli:latest
 ```
 
+#### `release-cli` for shell executors
+
+> [Introduced](https://gitlab.com/gitlab-org/release-cli/-/issues/21) in GitLab 13.8.
+
+For GitLab Runner shell executors, you can download and install the `release-cli` manually for your [supported OS and architecture](https://release-cli-downloads.s3.amazonaws.com/latest/index.html).
+Once installed, the `release` keyword should be available to you.
+
+**Install on Unix/Linux**
+
+1. Download the binary for your system, in the following example for amd64 systems:
+
+  ```shell
+  curl --location --output /usr/local/bin/release-cli "https://release-cli-downloads.s3.amazonaws.com/latest/release-cli-linux-amd64"
+  ```
+
+1. Give it permissions to execute:
+
+  ```shell
+  sudo chmod +x /usr/local/bin/release-cli
+  ```
+
+1. Verify `release-cli` is available:
+
+  ```shell
+  $ release-cli -v
+
+  release-cli version 0.6.0
+  ```
+
+**Install on Windows PowerShell**
+
+1. Create a folder somewhere in your system, for example `C:\GitLab\Release-CLI\bin`
+
+  ```shell
+  New-Item -Path 'C:\GitLab\Release-CLI\bin' -ItemType Directory
+  ```
+
+1. Download the executable file:
+
+  ```shell
+  PS C:\> Invoke-WebRequest -Uri "https://release-cli-downloads.s3.amazonaws.com/latest/release-cli-windows-amd64.exe" -OutFile "C:\GitLab\Release-CLI\bin\release-cli.exe"
+
+      Directory: C:\GitLab\Release-CLI
+  Mode                LastWriteTime         Length Name
+  ----                -------------         ------ ----
+  d-----        3/16/2021   4:17 AM                bin
+
+  ```
+
+1. Add the directory to your `$env:PATH`:
+
+  ```shell
+  $env:PATH += ";C:\GitLab\Release-CLI\bin"
+  ```
+
+1. Verify `release-cli` is available:
+
+  ```shell
+  PS C:\> release-cli -v
+
+  release-cli version 0.6.0
+  ```
+
 #### `script`
 
 All jobs except [trigger](#trigger) jobs must have the `script` keyword. A `release`