Clarification regarding Docker perms on Linux

[Spotlightsrule]

By default, non-root user accounts that are not in the "docker" group lack permissions to manage Docker. This commit addresses this fact, making it easier for newbies to get started.

[belden]

@scottyhardy FYI this very issue tripped me up, and the helpful link to the stackoverflow response which this PR introduces allowed me to begin using docker-wine. This seems like it's worth merging.