enter_chroot: add a note for users to ssh_config import

Make it clear to devs that edits to the copy inside the SDK will be
lost, and provided tips for how to change behavior dynamically.

BUG=None
TEST=`cros_sdk` has the updated file & comment block

Change-Id: Ib5e7818a72069c5160eb5ad069b68519be3ef531
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3244430
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
1 file changed