Improve documentation about PostgreSQL configuration and node version

See https://github.com/CartoDB/cartodb/pull/341

Signed-off-by: Sandro Santilli <strk@keybit.net>
CDB-1455 #resolves Only show one warning message when the user is close to the size & table limit
Merge remote-tracking branch 'origin/CDB-1455' into release/2.8.1