[Docs] Add installation methods

[AoMe · 青目]

Motivation

I am using selene in Arch|MacOS|Windows and I use ansible to setup my machines.

• For MacOS I use: brew install selene website reference
• For Arch I use: pacman -S selene website reference
• For Windows I use: scoop install extras/selene website reference

Note

I don't know the situation for other OS like Ubuntu and such.

Any information about that is appreciated.

Objective

It will be nice to have these kind of installation methods up on the installation section.

Adding something like this:

MacOS installation

brew install selene

Note

I don't know how are the docs generated exactly, but if you tell me, I can do a PR to make the changes.