Write a guide on how to add support for new platforms

[ferki]

A content idea is to describe how to add support for new platforms, like how to tell Rex to do the following:

• bare minimum to run commands

  • connect (optionally non-SSH)
  • detect the shell
  • execute commands on that shell

• manage systems

  • detect the OS
  • install packages on that OS
  • manage services on that OS

It should also be possible to do all that in external modules, not just within the core.

[ferki]

This is now considered as a content for the contributing guide, and superseded by RexOps/Rex#1395.