Added supported OS

Author: ken-matsui

pages/docs/installation.mdx

@@ -2,6 +2,12 @@
 
 [![Packaging status](https://repology.org/badge/vertical-allrepos/poac.svg)](https://repology.org/project/poac/versions)
 
+### Supported Operating Systems
+
+|                                                                      Linux                                                                       |                                                                      macOS                                                                       |
+| :----------------------------------------------------------------------------------------------------------------------------------------------: | :----------------------------------------------------------------------------------------------------------------------------------------------: |
+| [![Linux](https://github.com/poac-dev/poac/actions/workflows/linux.yml/badge.svg)](https://github.com/poac-dev/poac/actions/workflows/linux.yml) | [![macOS](https://github.com/poac-dev/poac/actions/workflows/macos.yml/badge.svg)](https://github.com/poac-dev/poac/actions/workflows/macos.yml) |
+
 ### Homebrew (macOS & Linux)
 
 ```sh