Adding CentOS 5.7 description

Update the build description on how to build Couchbase server
for CentOS 5.7

Change-Id: Iee0fb02755c43ec926edb22e6118b29243f71d46
Reviewed-on: http://review.couchbase.org/42513
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>