[DOC] Explain how to install

[tbouffard]

From CRAN: #10
From GitHub for latest unreleased/development version or for alternative

See https://cynkra.github.io/dm/#installation for instance

https://www.rdocumentation.org/packages/remotes/versions/2.4.0/topics/install_github

• Install from github while no package available on CRAN [FEAT] Initialize the project #21
• install from github for development [DOC] Explain how to install a specific version #70
• install from github with a specific tagged version [DOC] Explain how to install a specific version #70