When working on PR #134 about special env var being reserved, we had a few discussion about where we should define the ENV variables. It seemed redundant to have them at the command section.

Few points were raised on where they could be documented:

• linking to https://devfile.github.io/devfile/assembly_making-a-workspace-portable-using-a-devfile.html#_adding_commands_to_a_devfile in the spec
• linking to a new document about special variables in the spec
• a better place in the devfile spec