Add description for Debian-7

Change-Id: I79f4e3a7e3ec6f3154abc3a0c854c6943758c898
Reviewed-on: http://review.couchbase.org/40289
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>