Add google-perftools to the README for MacOSX

google-perftools need to be installed otherwise the linker won't find
tcmalloc.

Change-Id: I89a4422d31a2f7f9535cd100eaddf283c6d81223
Reviewed-on: http://review.couchbase.org/31777
Reviewed-by: Philip Labee <philip@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>