Add document pipectl install method using asdf (#4185)

docs/content/en/docs-dev/quickstart/_index.md

@@ -18,6 +18,7 @@ Note: It's not required to install the PipeCD control plane to the cluster where
 
 #### 1.1. Installing PipeCD client
 
+##### Method 1: Official Installation
 The official PipeCD client named `pipectl` can be installed using the following command
 
 ``` console
@@ -37,6 +38,12 @@ You can also move the pipectl binary to the $PATH for later use
 sudo mv ./pipectl /usr/local/bin/pipectl
 ```
 
+##### Method 2: [Asdf](https://asdf-vm.com/) Supported Installation
+
+```console
+asdf plugin add pipectl && asdf install pipectl latest && asdf global pipectl latest
+```
+
 #### 1.2. Installing PipeCD's components
 
 We can simply use __pipectl quickstart__ command to start the PipeCD installation process and follow the instruction

docs/content/en/docs-dev/user-guide/command-line-tool.md

@@ -41,6 +41,29 @@ You can use pipectl to add and sync applications, wait for a deployment status.
     pipectl version
     ```
 
+### [Asdf](https://asdf-vm.com/)
+
+1. Add pipectl plugin to asdf. (If you have not yet `asdf add plugin add pipectl`.)
+    ```console
+    asdf add plugin pipectl
+    ```
+
+2. Install pipectl. Available versions are [here](https://github.com/pipe-cd/pipecd/releases).
+    ```console
+    asdf install pipectl {VERSION}
+    ```
+
+3. Set a version.
+    ```console
+    asdf global pipectl {VERSION}
+    ```
+
+4. Test to ensure the version you installed is up-to-date.
+
+    ``` console
+    pipectl version
+    ```
+
 ### Docker
 We are storing every version of docker image for pipectl on Google Cloud Container Registry.
 Available versions are [here](https://github.com/pipe-cd/pipecd/releases).