Add note about paths to docker section of README

Right now, the docs make no mention of needing to pay attention to
where the user is running the supplied docker create command. This
can confuse users if they are not familiar with how docker create
works or what its arguments and options mean.

This will correct that lack of direction while preserving the
current workflow based on relative paths. I felt it was too
disruptive and close-minded to direct users to clone to a
specific path, as everyone has different prefs for their
local machine.

Signed-off-by: Cullen Taylor <ctaylor@gitlab.com>