dim: clarify dimrc a bit

Some notes I took while ramping up Shashank on dim.

Aside: Requiring that DIM_REPO is already checked out seems to confuse
too. Maybe we should just fall back to initializing an empty git repo
- we'll all all the branches and remotes anyway.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>