Add conda-forge installation instructions to README

[matthewfeickert]

There is now an air conda-forge feedstock https://github.com/conda-forge/air-feedstock

Name	Downloads	Version	Platforms

which means that people can install air now either in a particular project with

$ pixi init example && cd example
$ pixi add air
$ pixi run air format <path>

or install it globally (in an isolated environment with exposed API)

$ pixi global install air
└── air: 0.9.0 (installed)
    └─ exposes: air
$ air --version
air 0.9.0

or in a temporary environment with

$ pixi exec air <subcommand>

It would be great if this could get added to the README as well.

cc @salim-b

[salim-b]

Also via mise 😊:

# per-project
mise use conda:air

# globally
mise use --global conda:air

[matthewfeickert]

Thanks @DavisVaughan!