MB-6972:  add README to linux_repos scripts

Change-Id: Ia9b49d8cbaff07f36702b5c3427b8fb6cb626d61
Reviewed-on: http://review.couchbase.org/41150
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>