Add the installation guide with aqua to the document

[suzuki-shunsuke]

Hi, thank you for your great project!

We suggest to add the installation guide with aqua to the document.
aqua is a declarative CLI Version Manager.
You can install this tool with aqua.

aqua init # Create aqua.yaml
aqua g -i Songmu/ghg # Add this tool to aqua.yaml
aqua i # Install tools

For the detail of aqua, please see the official document.

We expect you can just copy and paste the following guide to your document.

---

Install with aqua

You can install this tool with aqua.

aqua g -i Songmu/ghg # Add Songmu/ghg to aqua.yaml

---

Thank you.

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