Add installation with gah (#4820)

- **PR Description**

This PR adds to the documentation a way of installation using
[gah](https://github.com/marverix/gah/).

> gah is an GitHub Releases app installer, that does not require sudo.
It is a simple bash script that downloads the latest release of an app
from GitHub and installs it in `~/.local/bin`. It is designed to be used
with apps that are distributed as a single binary file.

Author: stefanhaller

README.md

@@ -270,6 +270,14 @@ scoop bucket add extras
 scoop install lazygit
 ```
 
+### gah (Linux and Mac OS)
+
+You can install `lazygit` using [gah](https://github.com/marverix/gah/):
+
+```sh
+gah install lazygit
+```
+
 ### Arch Linux
 
 Packages for Arch Linux are available via pacman and AUR (Arch User Repository).