chore(readme): update instructions for nightly, add note on cleanstart
chore(readme): update instructions for nightly, add note on cleanstart

Instruct developers to download/use the latest MarkLogic 8.0 nightly.

Make mention of cleanstart.sh script for detailed steps which are being
tested daily and expected to provide a working middle tier, with caveat
that the script will only be functional on OSX.

laurelnaiad/marklogic-samplestack,ryanjdew/discovery-app-w-samplestack,laurelnaiad/marklogic-samplestack,dmcassel/marklogic-samplestack,omkarudipi/marklogic-samplestack,laurelnaiad/marklogic-samplestack,omkarudipi/marklogic-samplestack,ryanjdew/discovery-app-w-samplestack,supriyantomaftuh/marklogic-samplestack,marklogic/marklogic-samplestack,supriyantomaftuh/marklogic-samplestack,dmcassel/marklogic-samplestack,omkarudipi/marklogic-samplestack,dmcassel/marklogic-samplestack,marklogic/marklogic-samplestack,supriyantomaftuh/marklogic-samplestack,laurelnaiad/marklogic-samplestack,omkarudipi/marklogic-samplestack,supriyantomaftuh/marklogic-samplestack,marklogic/marklogic-samplestack,ryanjdew/discovery-app-w-samplestack,marklogic/marklogic-samplestack,dmcassel/marklogic-samplestack,ryanjdew/discovery-app-w-samplestack